Vial labels carry fewer facts than people read into them. It is worth being precise about what each number is, because half the dosing errors I have seen start as a misread label.
The milligram figure is mass of powder, not a dose and not a concentration. A 5 mg vial contains 5 mg. It says nothing about how much you will inject, and nothing about mg per mL, because there is no liquid in it yet.
There is no concentration on a lyophilised vial. Concentration only exists after you add diluent, and it is entirely determined by how much you add. If a label appears to state a concentration on a powder vial, it is stating an assumption about a diluent volume, and you should find out which one.
Volume markings on the glass are capacity, not contents. A 3 mL vial holding 5 mg of powder is a 3 mL vial. That is a container spec.
Batch and expiry apply to the powder. The expiry on the label is for the unreconstituted product stored as directed. Once you add water, a much shorter clock starts, and that clock is yours to track because nothing on the label knows when you opened it.
So the useful move on receiving a vial is to write the things the label does not say: the date you mixed it, the diluent volume you used, and the resulting mg/mL. Three numbers on a strip of tape. I derive the third with a reconstitution calculator rather than in my head, because it is the one that everything else depends on.
If a term on a label is unfamiliar rather than just terse, the glossary is a faster check than searching the product name.