We noticed some of our players have a “P” in either the Ref of Score column on our tournament roster. What does the “P” stand for?

The "P" stands for "Pending".  Players that have a "P" for their status have registered for a clinic from within their member record.  The clinic could be an in-person clinic or the online clinic.  At the time the roster was produced, those players had not completed their clinics (or the results of the in-person clinic have not been processed yet).  As soon as the player completes their online clinic or the office receives results from the in-person clinic they attended and updates the records, their status will go to "Current" and you will see a "Y" in the applicable column.  Players with a "P" for their status should NOT be performing those duties until they complete their clinics (they can still work as a Line Judge as that position does not require a certification).  Each team must show at least TWO players with a "Y" in the REF column, ONE player with a "Y" in the SCORE column (or a third in Ref column) and ONE adult with a "Y" in the REF column to be a legal team.  See other question in this category about why there is not a "Y" in both columns for most players.