In fact, they are usually added, or better designed, when two letters are joined as a single one. This usually happens when two letters have ascenders, like ff, ft, that clash together producing a rather unpleasant result.
Common-ligatures - no-common-ligatures
Discretionary-ligatures - no-discretionary-ligatures
Historical-ligatures - no-historical-ligatures
Contextual - no-contextual
Normal and none.
Each property can be declared with multiple values, containing all of the preceding declared values..
For example, if you like discretionary but not historical ligatures, you can write...