The Multiclass feature receives the following revision to Spell Slots and option for Level Restrictions.
You determine your available spell slots by adding together all your levels of classes with full spell progression (bard, cleric, druid, sorcerer, wizard, and any class that gains new spell slots at every level), half the total of all your levels of classes with half spell progression (artificer, paladin, ranger, and any class that gains new spell slots at every even level), and a third of the total of all your levels of classes with one-third spell progression (Arcane Trickster rogue, Eldritch Knight fighter, Hand of Faith rogue, Path of the Shamanic Adherent barbarian, Way of the Shugenja monk, and any class that gains new spell slots at every third level). Use this total to determine your spell slots by consulting the Multiclass Spellcaster table in Chapter 6 of the Player’s Handbook. Odd artificer levels are rounded up.
If you have more than one spellcasting class, this table might give you spell slots of a level that is higher than the spells you know or can prepare. You can use those slots, but only to cast your lower-level spells. If a lower-level spell that you cast, like burning hands, has an enhanced effect when cast using a higher-level slot, you can use this enhanced effect, even though you don’t have any spells of that higher level.
For example, if you are a paladin 5 / ranger 3, you count as a 4th-level character when determining spell slots: you have four 1st-level slots and three 2nd-level slots. However, you don’t know any 2nd-level ranger spells. You can use your 2nd-level slots to cast any of your 1st-level ranger spells or your 1st-level and 2nd-level paladin spells, and can benefit from any enhancement from casting the spell with a higher-level slot. For instance, when you cast longstrider using a 2nd-level slot, you can use the enhanced effect, despite not knowing any 2nd-level ranger spells.
Once you take a level in more than one class, you can’t increase the level of any class beyond 5th level until each class is at least 4th level.