A film composer usually is hired to write all  original dramatic music for your movie. Any scene that has “score music” is part of what the composer should do for your movie. Anything that goes beyond that needs special negotiation with the composer. This includes:

Source Music:

this is music that can be heard by the characters in the movie, for example on radio, a concert, an elevator etc. Usually, this is music that is being licensed from music libraries or other musicians. However, sometimes source music is written by the film composer as well, especially when it takes a dramatic role (for example source music morphing into score music or a tune on the radio that gets an important role in the movie). In such cases, this needs to be noted in the contract. Also, you should be aware that no composer could possibly be able to pull of any music style convincingly, so if you ask from your clasically trained, orchestra orientated composer to write a styllistically on the spot heavy metal tune to be played at a concert in the movie, it might not be the wisest idea and probably easier to just license it from somewhere else.

Songs:

It is usually not the task of the composer to write any kind of pop songs that are added to the movie (e.g. for montage scenes etc.). Usually these are licensed as well. If you want him/her to do that as well, mention it in the negotiations.

Recording/Mixing music:

Some composers do that but usually the recording and mixing of the film music is done externally under the supervision of the composer and therefore needs to be budgeted in. More on that in a later article.