For an item to separate between another element of the same type [מין במינו], and another surface: Rosh HaShanah 27b; Succah 37b [2x]; Bechorot 9b
Distinguishing where [s]he doesn't care about the presence of the separation, under ordinary circumstances: Shabbat 114a; Eruvin 4b; Pesachim 45a-b; Succah 6b
Distinguishing where the majority of a surface is not covered: Eruvin 4b; Succah 6b
Considering an item which is added to a Mitzvah-item solely for beauty, to be a separation when one is supposed to hold the actual item: Succah 37a
Whether to distinguish between a separation on one side, or both sides, of a garment/saddle-blanket: Shabbat 114a