The second number reflects the pitchers closer rating. You may be aware of the closer rules but for those that aren't:
If a pitcher has a closer rating of 0 then he is susceptible to tiredness as soon as he allows a hit or a walk if it is a closer situation. Since Mariano has a closer rating of 3, after he records 3 outs he'll lose his effectiveness as soon as he allows a hit or a walk. Once effectiveness is lost @ become SI**.
To your point of an R2 going 6 outs before getting tired, this is from
"How to Read a Pitchers Card" on the
Game Rules page.
A relief pitcher becomes fatigued under the same circumstances as a starting pitcher does (i.e. allowing 3 hits and/or walks within a 3-out period, etc.), but the point at which these rules kick in is determined by outs. A reliever with a Relief endurance of 1 is immediately vulnerable - he could become fatigued if he allows 3 hits and/or walks before recording a single out. A reliever with a Relief endurance of 2 would become vulnerable after he has recorded 3 outs.Here's a more detailed rules source:
http://www.baseballthinkfactory.org/btf ... mrules.htm