It's in the PHB.
For non-exclusive skills, your max rank for a skill that is a class skill for any of your classes is Character Level + 3. Your max rank for a skill that is cross-class for all of your classes is (CL+3) / 2. However, skill points bought with a level in a class for which a skill is cross-class are paid for at cross-class prices.
This is not the case with Exclusive skills.
Your max rank in an Exclusive Skill is your level in classes with access to the skill + 3, and you cannot purchase ranks with skill points from a class that has no access to the skill.
A Rog5/Ftr1 has a max Perform rank of 9. If he had 8 ranks before levelling in Fighter, it will cost him 2 points to get that 9th rank, since Perform is a cross-class skill for fighters.
A Rog5/Ftr1 has a max UMD rank of 8 : his levels in classes with access to the exclusive skill (Rog5) + 3. He cannot spend fighter skill points to purchase ranks in UMD.
-Hyp.