Bacteriostatic water

What it is

Sterile water containing a preservative, which is what allows a vial to be entered more than once rather than being a single-use preparation. The volume you add is a choice, not a fixed quantity: the same vial reconstituted with 1 ml or 3 ml holds the same total, at three times or one third the concentration.

Why the volume is arbitrary but the concentration is not

More solvent makes each unit weaker and the measurement easier to read on the syringe. Less solvent does the opposite. The total in the vial does not change either way, which is the point people most often miss.