What is the difference between cues and instrumentals?

[AlonMarcus]

I'm a little bit confused.
It seems these are two different creatures.
Sometimes there is a requirement that the cue will be 90 sec but instrumentals can be up to 4 minutes.

One of my rejections actually used this logic. The track was 2:15 and the reviewer said that cues were supposed to be 90 sec. Length was one of the reasons for rejection.
But how he (or she) could tell my track was a cue and not an instrumental?