Every Activity needs someone who is responsible for it. That carries with it a basic set of obligations. We call this the Activity Lead (AL). Anyone can be assigned the Activity Lead role (not only PMs). The Activity Lead is basically a DRI (Directly Responsible Individual).
In terms of managerial skillset, PM is a more difficult Role. That's why being a PM is a career. In addition to the basics that all Activity Leads must do, PMs Role specifically is the stuff above and beyond that, and additionally includes responsibility for: accurate budgets and forecasting, managing customer feelings, advanced risk management, complex planning, bad-customer management, more advanced progress tracking, invoicing, contractor expenses, upselling, and much more.
Why do we have the distinction? Not all projects require a full blown PM. Some simpler projects can be led by non-PMs. These leaders do however still need to follow certain principles. These basic principles that every project needs, are extracted into the Activity Lead role. For example during our 2019 Krk Summit, Leon was the Activity Lead for "squad potato", this meant that he had to make sure we worked together, that there were "washers" and "peelers" and a "cooker" and that all tasks were distributed correctly and that the finished potatoes were ready on time and enjoyed by all; Leon did a wonderful job, but he's not a PM – there were no bad customers or budget forecasts to take care of here.
The default case at 9Y is that the same person takes on both the PM and the AL Roles.
It's also possible that a project splits the PM ad AL Roles across different people. For example the Engineering Lead could take the Activity Lead Role. This could happen if for instance the customer is a software engineer and the project is deeply technical, in which case having the Engineering Lead be the main POC (point of contact) is a more natural setup. This Engineering Lead would then have a PM in the background to support them with invoicing, planning and all that advanced project management stuff that they're not trained in.