Given that Product Marketing role definitions and skills vary so much by company, how do you develop a transferrable career path if you end up switching firms along the way?

Yes, PMM role varies a lot by industry and company. So you have to ask yourself what you want to do and what ultimately interest you. When you choose what companies, PMM groups to join, you need to evaluate whether that helps you get to where you want to be. For example, I am not into enterprise B2B. Product marketing in that space requires a lot more sales enablement work that I'm not passionate about. I've personally found the following background helpful to excel at product marketing:

• Hands-on channel marketing experience. A good PMM need to know how different channels work, e.g., how to do acquisition via paid marketing, how to do customer lifecycle management
• Classic CPG marketing training. Best place to learn about segmentation, targeting, positioning and brand management
• Strategy consulting. PMM needs to have a business mind. Strategy consulting is great at teaching you how to solve an ambiguous problem

No doubt your product, its value props and your audience will change from company to company. 

What doesn't change however is your job to be done as a product marketer, and the role you get to play. No matter what organization I've been part of, PMM has always sat at the intersection of product, sales and marketing. Now you might get pulled in one direction or the other depending on the company (see answer above) but on any given day, you are the only person this side of a GM that will be as close to all three of those functional areas. 

So my advice to you is to perfect your ability to service each of these three functions by building the muscle memory and pattern matching to meet and anticipate their needs. That's something that will help you navigate your next job, any job, should you choose to jump ship. 

Specifically: 

• For Product, become their trusted advisor. Learn and master how to support the product lifecycle end to end with market research to test product hypotheses, naming, pricing, packaging, positoning to GTM etc. Be prescriptive about what needs to get done for the product to succeed in market. Remember that while product and engineering may come up with the plot, product marketing delivers the narrative. And it has to be a two way conversation, because just like with movies, one does not work without the other.
• With Sales, especially in the enterprise, you need to be sale’s best friend. That means equipping them to have the best and most meaningful and authentic conversations with customers via killer assets and training programs.
• As for marketing, you’re the quarterback, the router of information and single source of truth when it comes to messaging. You provide the messaging primitives and content backbone as well as go/no-go guidance on which channels to activate and how.
  

In my career path I've looked for a few things and created a few career principles...

1. Finding growth industries that will be more important in the next 3 - 5 years given where I see trends moving...For example, I was in AdTech in 2010 when mobile and display advertising was just starting to take off, marketing automation in 2014 as marketers were starting to get much tighter around spend, measurement, and performance and finally, marketplace-based food delivery in 2016 just as on-demand services were reaching into more and more consumers lives! I wonder what will be even more important to people's lives in the next 3 - 5 years...

2. Identifying hard, ambiguous problems - These are the problems I am most passionate about solving! Solving the hard problems will differentiate you from a career perspective. It's great if you are working on Search at Google, but that's a problem that has probably been 80% solved, and probably won't be increasingly important in the next 3 - 5 years given current trends.

3. Finding great people and great products - Uber Eats taught me the value of finding a great product and working with great people. When I first joined Eats I was absolutely blown away by how smart, motivated, and talented people were. It felt like a company I just had to join (and haven't regretted it since!) It was also a product that I thought had a ton of potential. 

Those were just a few of my principles, but whether you're at this company or that company, I recommend developing the things you're trying to optimize for, posessing the growth mindset (there's always something to learn!) and finding hard problems.

Switching companies as part of your PMM journey is a good thing (if you're not jumping around too much). It will give you a broader perspective on different GTM channels, business models, org structures, sales processes, etc. You just need to make sure you are growing and acquiring new skills with every role. Even if you're making lateral moves, seek out new areas of responsibilities and more opportunities to do things you've never done before - maybe take on analyst relations or build the competitive intelligence program from the ground up.

This question reminds me of Reid Hoffman's book "The Startup of You". If you think of yourself as a startup, think about which skills you have and are good at, what others you want to learn, and see how you can hone on those skills in your current or upcoming role as well as which other skills you want to learn or develop so it will help you get the next role. There are different career paths for product marketers, so you have options.

PMM role definitions and skills do vary significantly across companies (and even teams within the same company!) but I see this as an opportunity. 

The main skills are still foundational across all roles: 

1. Having a deep understanding of the customer - being able to clearly grasp customer pain points and communicate them to the product team, and ensure you're solving for them with any messaging. This also includes being able to ramp up on a new space quickly and learn the nuances of a product/industry

2. Clear and structured communication - being able to succinctly communicate and articulate complex considerations clearly, both internally and externally

3. Working cross-functionally and often being the "quarterback" or glue across multiple functions

If you're able to excel at these key skills, no matter how much the role changes, you can still take these skills between them. Soft skills are more important to invest in, and even if a job at another company is defined slightly differently, proving you have stellar soft skills and can learn a new space/slightly different role quickly goes a long way. 

I guess I've never really thought of this as a problem. If you follow your passions and continuously take opportunities in great organizations, you will automatically create a career path that sets you up for the type of future opportunities you like and are passionate about.

If you're more concerned about progression, and moving into successively more senior positions, you're actually far better off switching companies with some regularity. Not to mention you'll make a lot more money over the long run.