[Jtsummers]

I'm not the biggest fan of UML everywhere, but one benefit of it is that certain diagram types are standardized.

https://en.wikipedia.org/wiki/Unified_Modeling_Language

Check out interaction diagrams for what you describe. Two parties (A and B) would be represented as two columns, moving down the diagram means moving forward in time. The arrow going from one to the other is a message (either an actual message or a function call or something). So A->B means A is signaling (the text and context describe how) B. If B is requesting information, you'd have two arrows. First B<-A, and then A->B with the response (attempted plain text version):

   t A     B
   0 |     |
   1 |---->| A sends data to B
   2 |     |
   3 |<----| B requests more information from A
   4 |---->| A responds

A and B could be people, processes, classes/objects, servers, whatever. The interaction points are described with the context of the diagram and text. (NB: t is not meant to be explicit here, I've included it to illustrate the passage of time going down the diagram.)

[AstroJetson]

If you just want a sequence diagram like that I suggest https://www.websequencediagrams.com/

It's now as a website, but I have the server version and use it at least once a week to crank out interactions