2 resources from sajithpai.com we point people to, and the questions each answers.
✍️ Essay
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Pai splits PMF into product to problem fit and then motion to market fit, and that second half is exactly what founders mean when they ask whether their GTM is ready to scale. He argues it through Indian companies (Atomberg, Urban Company, Meesho, Meru) rather than Silicon Valley examples, so the market dynamics look like the ones you are actually selling into.
The canonical essay, and the one that actually spells out different discovery and monetisation playbooks per segment rather than just sizing the segments.