@evan Well the main difference in definition would be that people only consider paid contract/employed experience as "professional" vs. unpaid hobby work.

And especially in the open source community you have a lot of projects that are done by unpaid volunteers, some potentially still in training or simply self-trought. And those definitely reach the level of "traditional" paid work so limiting "professional experience" in a way which excludes this work (which some still do) would mean that it's only "somewhat important" for software engineers to have paid experience.

But nonethelss the paid aspects, imo. it's still important to have experience in complex software projects beyond simply coding, but also project management and support skills but I wouldn't say that these can only be gained in what is commonly refered to as a "professional context".

@evan Depends on the definition of "professional experience" but definitely at least "somewhat" for me.