Temp (temporary) tracksare a great help for directors. And truly it may be very convenient for them to use a piece of music already written in order to describe to  other people involved in a project - composers for example - what they have in mind. 
Also temp tracks can be used at different stages during the production of a film - for example for screening purposes - when the music is not yet written. 

But in the words of Fred Karlin 'no phrase, in the language of  film, can creates as much anxiety or even anguish than temp track !'. From a composer point of view,temp tracks create a very serious problem: people, directors, producers, editors, etc..., might get used to them after hearing them all the time, even though, in the end, they may not be suitable. 
A better alternative would be to get the composer involved much earlier on in the project and try and describe exactly what kind of atmosphere you want the music to evoke. Maybe even illustrating this with examples from other films. 
A good composer will be fully conversant with the styles of  major directors and genres, and should be able to help here in case if you are really stuck. 

Underscoring or scoring refers more precisely to the process of putting music to a film after the film has been shot. When music is recorded before we have ... prescoring. Obviously prescoring is a necessity when the pictures have to be synchronized to music, which, of course, is the case in musicals - where actors can hear the playback and mime the action. 

But, in general, what everybody calls film music is underscore, and it is one of the basic elements or building blocks available to a director to tell a story on a par with lighting, sets, camera works, dialogue, etc... And this, of course,  implies music specially written for a particular film. Who doesn't remember the music of The Mission, Paris, Texas, Shaft,  Psycho, Total Recall, The English Patient, etc... 

Source music is the actual music that is occurring in the film itself, coming from a car radio, a jukebox or in a nightclub for example.