What I actually asked for when requesting a co-author's letter about our joint work

Had significant collaborative work with one specific co-author, and asked them for a letter specifically addressing our joint project — different from a general reference letter, since I needed something that clearly delineated our respective contributions. What I asked for specifically: a description of the project itself, and then explicit, specific language about my individual role versus theirs — not "we worked on this together" in a way that leaves the division of labor ambiguous, but a real accounting of who did what. This mattered a lot for the original-contributions criterion, where I needed to establish my own specific, individual contribution clearly, not just my participation in a group effort. The letter that resulted was more useful evidence than a generic collaboration acknowledgment would have been, precisely because it did the work of clearly separating my individual contribution from the collaborative whole — exactly the kind of specificity other posts here have pointed to as the difference between a testimonial and real evidence.