Takže co sem se dočetl na glemax fóru:
The PHB gives an example explaining this under Multiclassed characters, page 60. Lidda as a level 4 Rogue adds a level of wizard:
"She could spend some of her 4 skill points on Rogue skills, but, since they would be treated as cross class skills for a wizard, these skill points would each buy only one-half rank"
So if you are say a level 4 Rogue as the example way, and you have 7 ranks in Move Silently, and you add a level of Wizard, then can spend your points on Move Silently to reach maximum ranks in Move Silently (8 ranks) however as a Wizard Move Silently is a Cross Class skill so it would cost 2 Skill Points to increase that rank.