Lets say I want the same bass part for my entire song and I want to only record 2 or 4 bars (to keep it tight) and then stretch / copy it so it fills the entire song/track. (or copy the 2 bars and insert them wherever I like).
If you drag the arrange entry (your 2 bar bass line) with the ALT key down, it will copy, or if you hold ALT when you drag the bottom right corner of the arrange entry it will copy as many times as it needs to fill the bars you have dragged it.
With all these methods the part must be exactly the right length though. If you need successive parts to overlap (which is quite possible if you don’t want to abruptly cut off the last note in the part, assuming we’re talking audio not midi) then you’ll have to find another way. With a well defined starting point of the part you can just use snap to allign all the parts to the grid.
If you size the single event to an even bar lenght and then duplicate it n times it’s an easy task to select them all afterwards and adjust the start and/or end togheter and then crossfade.