Access another company's subsrciption plan

“Pool of licences”.

A company can buy X licences of a product, like Creative Cloud all apps and any other subscription, that you can also buy individually, except the Photography plan. The plan administrator then assigns licences to people by inviting them to join via their e-mail address. You can also invite someone without a plan assigned or as administrator.

For stock, the company takes a plan, like the 750 assets/month. Every person having assigned a plan has access to that stock plan and eats plan credits when licensing items. There is to my knowledge not possibility to limit this. There may be with an Enterprise subscription, I'm not very familiar with that one.

To have access to the stock plan, you need to have a plan assigned, or you must be an administrator. You do not allocate a stock plan explicitly to specific users. The stock plan by itself counts also as a plan, so it can be, and needs to be assigned to a user. 

This looks to me like an ad hoc solution for making stock available to Teams and Enterprise, without breaking the rule to be able to assign a plan only to one ID. But if you have an enterprise with 5 Creative Cloud plans and 1,000 Acrobat plans, that creates a problem. You probably don't want those Acrobat users eating up your stock credits, just be fiddling around.