why are chromatids called as sister or daughter chromatids. at some case u call as sister chromatids and at some cases u call as daughter chromatids please explain?

When a single chromosome replicates or duplicates its genetic material to generate two copies of itself, then each part is called a sister chromatid. They are held by the centromere.

During cell division, anaphase, when these two are separated from each other and move to opposite poles then each single strand is called a daughter chromatid.