fair-dealing is prescriptive, fair-use is not
Copyright Acts in fair dealing jurisdictions enumerate what uses of copyright material are not infringements if the material is dealt with fairly (generally without actually defining fair dealing). For example, in australia, fair dealing can be argued if the use is for:
You don't give a jurisdiction, but from the DMCA tag I'm assuming you are interested in the USA.
There is no hard rule about what counts as "fair use". The law lists 4 factors for courts to take into account:
The purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes;