Please refer following documents if helps
Pricing Example: - Applying Discounts to a Promotion Modifier (Doc ID 392257.1)
QPXPRMLS : How to use Promotional Modifier as a Qualifier for Another Modifier (Doc ID 287509.1)
Order Level Modifier Is Not Applied For Orders With Promotional Goods While Booking (Doc ID 747409.1)
How to Give One Promotional Item Per Sales Order (Doc ID 1487247.1)
1- The link modifier can be Manual or Automatic. That depends on the flag Automatic at the Modifier Header or Line Level.
Manual, means that the user has to do an action to apply the modifier (if it is allowed)
2- Qualifier: that means how can have the benefit of the Modifier. In both cases Automatic or Manual. QP will allow to apply only the modifiers which have the "good qualifier".
The Modifier application hasn't any direct link with the price list.