In principle, timing can be linked to any musical structural concept. The most concrete of those are the following.
Although the most obvious metrical units are bar and beat, this strictly hierarchical structure may extend above and below these levels. Special expressive marking of the first beat in the bar, either by timing, dynamics or articulation, is a common phenomenon (Sloboda, 1983).
Phrases may not be ordered in a strict hierarchy, and may cut across metrical structure. Phrase final lengthening is the most well-known way in which they are treated (Todd, 1989)
A large proportion of the timing variance can be attributed to rhythmical groups (Drake & Palmer, 1990). Some standard rhythmical patterns, like triplets, seem to have a preferred timing profile (Vos & Handel, 1987).
Small timing asynchronies within a chord (called chord spread) are perceived as an overall timbral effect - the actual timing pattern is hard to perceive.
Ornaments, like grace notes and trills, can be divided in acciaccatura, so called timeless ornaments, and appoggiatura, ornaments that take time and can have a relatively important harmonic or melodic function. The former normally falls outside the metrical framework, the latter tends to get performed in a metrical way.
The independent timing of individual voices is sometimes hard to perceive because their components are immediately organised by the perceptual system in different streams (Bregman, 1990). This is not the case with (almost) simultaneous onsets that result in clear timbral differences. This can be heard in ensemble playing where often the leading voice takes a small lead of around 10 ms. (Rasch, 1979).
Any associative relation, e.g. between a musical fragment and its repetition, can be given intentional expression by using the same or different timing patterns.