Clocktime representation - a representation where timing (duration, onset) is represented by some real number and could support some notion of e.g. scaling.
The clock may commonly be seconds but it could be e.g. MIDI ticks.
Metrical representation - a representation where timing (duration) is represented by a symbolic value denoting a metric unit (e.g a quarter note) possibly dotted.
A metrical representation, like staff notation, can be thought of as tiled - no gaps (rests are concrete objects within the representation), no overlaps. Onset is implicit due to tiling. A metrical representation is expected to easily support concatenation.