The specs would be how the communication between the servers is supposed to happen. Like
Server A sends a “hello?” message to server B and serve B will respond with "hello."
Sever A then sends a message “I’m server A” and server B will respond “prove it” so server A will send proof via IDProof spec v1.2 and server B will say "ok A, how can I help?"
Server A will send "please me user comment 12345” and server B will respond with "comment 12345 was posted by user Skroob on 1/1/2020 and says ‘I have the same combination on my luggage!’
So the complaint would be like "we used IDProof spec v1.2” and the serve said “no that’s only valid on prime number Thursdays!”