I work on low(ish) latency trading systems in FX. FIX is the standard communication protocol and familiarity with it is essential for me. Here you can look up the standard message types and tag values: https://fiximate.fixtrading.org/
They also have docs for the standard message flows you can expect during trading. I use it regularly.
Also work in the FX space and could never find a good FIX specification resource. Always had to rely on whatever the broker shared with us, but it always felt incomplete. This is a super useful resource. Thanks for sharing!
Do you really need anything more than the dictionary? https://www.onixs.biz/fix-dictionary.html
Implementations varies, no one forces the third party to use message types 100% as intended.