In the Name of God, Most Merciful and Compassionate
After first writing about water, tanning hides and bones, using artifacts, and the toothstick, the author of The Ultimate Conspectus moves to ablution. He mentions its obligatory actions and its recommended actions.
The obligatory actions of ablution are actions that must be performed for the ablution to be valid. The ablution will not be valid if any of these actions are omitted.
Recommended actions are actions that should be performed but could be omitted. The ablution will be valid even if all of the recommended actions is omitted.
It is useful differentiate between obligatory actions and recommended actions because sometimes we do not have enough water or enough time to make a complete optimal ablution. Knowing which actions are obligatory and which actions are recommended allows us to omit non-obligatory acts when there is a need to do so. It also lets us know whether an ablution is valid if something has been omitted.