I think what you're looking for is "measure words". Any noun representing a count or size or amount is called a measure word. So you can say "slice" and "jar" are measure words, and "a slice of" and "a jar of" measure word phrases.

A "slice" of bread could also be a "piece" of bread. But there's not another good way to say "jar." You could call it a "container", but that's much less specific.

A slice or piece is a unit. A jar is a container.