On a day not legislatively reserved for one obligatory fast, several valid types of fasting may be possible: make-up, expiation, vow, or voluntary fasting, subject to their respective conditions.
This possibility of competition explains why an undifferentiated intention to fast does not necessarily identify a particular outstanding obligation.
For example, if someone owes both a Ramadan make-up fast and an expiatory fast, which duty is discharged by a valid day of fasting? The answer depends on the legally effective intention and applicable conditions, not simply on the passage of the day.
Separate requirements such as continuity in some expiatory fasts must also be respected.