The Recipient Cap* is an amount that you can set to put a limit on the earnings that a recipient can receive for a track.
This feature allows you to ensure that you only pay a collaborator a specific amount of a song's earnings. As soon as the Recipient Cap is met, that person will no longer receive a percentage of the song's earnings. This is useful in situations where you promised to pay someone for their help in the production of a song (ie: A Work for Hire agreement, etc) but rather than paying them up front, you can pay them from a percentage of your song's earnings up to the amount that you set for their Recipient Cap.
If you decide that you owe Artist X $750 for their work helping you create a song, you can set their Recipient Cap to $750, then Artist X will start being paid whatever percentage you decide of that song's earnings until Artist X earns $750 (The Recipient Cap). As soon as Artist X earns $750 (the limit on the amount you wanted to pay them) for that song, the percentage you were paying them for the song reverts back to you, the account holder, and you would then be earning 100% of any future earnings.