As a software developer, what information do you want from your clients?

I'm taking over as the lead of a software development project for a corporate client to build a bespoke project management software (Probably in MS Dynamics 365). As part of this, I will need to liaise with third-party software developers in order to build the functionality they need. I am not a software developer myself, so my question is what information do software developers look for/really need when working as a third-party developer for corporate clients?

Process flow diagrams seem to be the go-to option that I have seen so far, but there still seems to be a lot of miscommunication in terms of how we need the software to function and how it is delivered.

When working as a third party, do you ever think "Why can't they just provide X, Y & Z so I can actually do my job"? And if so, what are X, Y & Z?

Thanks in advance for any suggestions of areas or resources to consider.