The way to keep track what Percentage group (Lord, Hero, Core, etc) a models points count against is simple.
"If the points paid are under the Unit entry that is the group points it counts as"
Say one buys a Lord with X points of magic items and Y points on a mount. All of those points go against the 25 percent lord cap.
The only time points don't count towards any Percentage group is when it is a core unit that in its entry says, "Does not count as Core." Vampire Counts are rife with these units.