Yes, that's an appropriate way of understanding a role!

The nuances come when you need to decide how granular or broad a role should be, or simply when the work is unique enough that there is no clear 'industry standard' for how to split the work... but yes, as a general rule of thumb, a role is a "business function".

We noticed it varies and also changes a lot, we have a lot of circles and roles that are not so much a business function as a target group or part of our product. It also goes back and forth over the years as our strategy changes.

I also see roles as business functions. They can scale to as high level, large, or granular and small as you like. Even roles can "evolve" or mature over time, growing, and differentiating, and then splitting, or combining. 

I have found that a uncovering of the work not related to positions but from a day to day week to week and month to month process works really well we create a key people brainstorming workshop where we walk through each persons typical day, week etc and really just capture the work. We then go the work into logical groups / role formation but still keeping it fairly loose this is then presented to the group / proposed circle to review in another brainstorming session have we grouped the work appropriately this creates the initials roles and the work quite often is then either covered under an overarching purpose or where the "work" is required as input or output for another role and accountability is defined.

what I have found in my experience is the discovering the work process has dramatically streamlined the initial backlog and tack up of governance/Holacracy whereas when we have started by mapping initial position descriptions / functions there is a much bigger disconnect from reality to the starting structure