You must spend a minimum of 25% of your points on Core units.

Core units are the heart of your army, the iconic troops who make up the bulk of every warband and warhost. Unlike other types of unit, there is no maximum to the proportion of your points that you can spend on Core units.

Some Core units do not count towards the minimum points you must spend on Core units (sometimes written as 'do not count towards the minimum number of Core units you must include' or variations thereof) or indeed the minimum number of units you must include in your army. In fact, such units don't count towards any category, just the points value of the army.