There are three kinds of dependencies with respect to the reason for the existence of dependency:

1. Causal (logical)
   • It is impossible to edit a text before it is written
   • It is illogical to pour concrete before you dig the foundations
2. Resource constraints
   • It is logically possible to paint four walls in a room simultaneously but there is only one painter
3. Discretionary (preferential)
   • I want to paint the living room before painting the dining room, although I could do it the other way round, too