A381 additional
Everything Cieyrin said is true, but he missed one minor thing I'd like to add. When setting the CL of an item to a lower CL than yours, you can't lower below minimum for whatever spells and such are requisites for the item.

Example: A level 12 Wizard is brewing a Potion of Haste. He could choose to brew the potion at any CL between 5 and 12. 5 is the minimum level needed for him to cast Haste, 12 is his own CL. Making it with a higher CL would lengthen the duration of the spell, while a lower CL would cost less.