The optional cap rule - Pg 61 SR4A
QUOTE
Optionally, gamemasters may choose to cap dice pools (including modifiers) at 20 dice, or at twice the sum of the character’s natural Attribute + Skill ratings, whichever is higher.
This would be 2*(Attribute + Skill)
The social modifier cap, which is completely different. Pg 130 SR4A
QUOTE
Cumulative positive Social Modifiers may not exceed the character’s combined natural Attribute + Skill Ratings.
So when doing a social test with the optional cap rule, calculate positive social modifiers, apply that cap, add in dice pool and negative social modifiers, and then apply the second cap.
Note that for hacking (which throws logic under the bus) and rigging tests (Pg 247 SR4A) which throws everything else under the bus, your table will have to decide what attribute is important or if the optional cap rule still applies, or if you actually use the equipment's stat to determine the cap. For Hacking, logic still seems the logical choice. For Rigging, I'm very tempted to go for Reaction while jumped in and Logic while using command.
As far as I know, RAW (and Missions) has completely ignored the combination of the optional skill cap rule and hacking/rigging. If someone can find me a rule, I'd appreciate a page number and book.