But what are those capabilities? That’s an undefined thing here since I don’t know what your installer does (not) support. It seems you’re asking for the PEP to define what potential capabilities an installer would need to have and then list those required capabilities somehow. You don’t have this with requirement files today either (but maybe you wish you had this?).
That’s fine and between you and your locker.
Sure, but users are not generally doing any of that day-to-day.
Right, so I still don’t know how to take that. 
Is another tool encroaching on pip’s territory and gaining traction a good or bad thing?
I wouldn’t read into the resolution behaviour too much; assume a proper resolver.
That’s between you and your locker just like it’s between you and pip today.
I’m not specifically, but it also requires innovating even more to support it as I don’t know any tooling that directly supports it short of creating a “lock” file that is very much tied to your platform and the exact files you installed (which is fine and possible with this PEP).
Not sure, but I don’t see any reason not to support this use-case just because pip doesn’t support it. In the end it’s bits off the wire, so providing out-of-band info so those bits can be interpretting appropriately doesn’t seem like a bad thing.
OK, I’m going to flat-out ask: what do people want here?
There’s the “give me requirements files” which still requires running a resolver and doesn’t really “lock” in a traditional sense, but it does restrict what is considered at installation time and allows for the potential cross-platform dependencies files that PDM and Poetry have found successful.
Then there’s the “give me requirements files, but w/o needing a resolver” which basically means a traditional lock file which doesn’t require a resolver (at most marker resolution), but which inherently means the lock file is platform-specific; what pip freeze/pip-tools found successful.
We tried to come up with something that services both needs as you can view the more flexible PDM/Poetry solution having a stricter subset to cover the pip freeze/pip-tools solution. To me, it seems to have failed based on the reaction we are getting (at least in its current form).
I have only one full rewrite left in me on this PEP (unless @pradyunsg and @uranusjr have more 
), so I am now asking all of you to vote on what you want. I can then have a think on the topic and make a file format proposal we can iterate on and then based on the agreed-upon format, update the PEP.
- Traditional lock file (i.e. no resolver necessary;
pip freeze/pip-tools) - Basically pip requirements file (i.e. resolver required; PDM/Poetry)