left-over photopolymer resins
Considerations for storing resin include using a light-tight bottle, preventing stray ultraviolet radiation from prematurely curing the resin.
You'll also note that users will filter the resin through a fine mesh filter. I've seen some videos in which the user pours through coffee filters to remove as many particulates as possible.
Left-over resin that has been carefully filtered is effectively the same as fresh-from-the-bottle material, from a purely technical view, but you can pick up contaminants that bypass the filter.