4.1.2 a) states that: 

Other Circle Members may ask you to process any Accountability or Project on a Role you fill in the Circle. If you have no Next-Actions tracked for it, you must identify and capture a Next-Action if there are any reasonable ones you could take.

Does this imply that the role-filler has to accept the task even if they have no capacity? Or can the role-filler also discard the request based on the reason that he/she currently has no capacity.

At the moment I generally assumed that a role-filler has to take on a project if he/she finds that it reasonably fits purpose & accountabilities but can de-prioritize it if conflicting tasks exist.

In the concrete tension we have a developer role filled by multiple people. This role generally contributes to different IT-projects. The project manager role might ask/tell the developer to take on more tasks (beyond capacity ca. 40hrs per week) and try to achieve a high prioritization.