Impact is typically measured by customer and business value that product/project brings in. First step is to understand customer scenarios/workflows and associated pain-points and needs. Combine that with business opportunity of that investment, also having a close eye on the market and compete landscape. Finally, use rigorous prioritization to focus on highest ROI - take into account both customer/business value as well as associated cost to deliver. This is no different from a typical Product team. A key focus on platform team should be to invest in durable, long-lasting capabilities that will benefit many customer as opposed to focusing on short-term wins for a select set of customers (this can result in custom solutions that become hard to generalize in future).