BBM uses the actual current-season MLB player stats in its game resolution. No stat can be used more than once. If a MLB performance is not used live in your BBM game it is stored in a stat pool for use later in the season. Stat pools are accessed when your player doesn't play that day in MLB (or in Ultimate Leagues when he faces a different handed SP than your BBM game).
It is useful to keep track of your stat pool to know what stats are available. You can view this information (as well as that of your opponents) on the Roster. Click Roster on the Team menu.
The roster page has a dropdown menu to select the team to view and one called Show Stats. You can select which stats to view (ALL, USED, REMAINING and SEASON).