Three things that are not the same
A needle order needs the system, the gauge and the finish, and these are routinely confused. The system describes the needle's geometry and its designation — it is what makes a needle fit a particular machine's cam track and knock-over. The gauge describes how many needles occupy a given width. The finish describes the surface treatment. Two needles can share a gauge and be entirely incompatible.
Where the information actually lives
In order of reliability: the machine's own documentation; the machine nameplate; the packaging of the last set you fitted; the marking on the needle itself. The packaging is often the most useful and the first thing thrown away — if your store has an old box, photograph it before it goes.
When the marking has worn off
A needle that has been running is often unreadable. In that case the machine brand, model and gauge will usually narrow the possibilities to one or two candidates. Send a photograph of the nameplate and of the old needle beside a rule; the overall length and the hook geometry are usually enough to confirm.
What to send us
Machine brand and model from the nameplate, the gauge, the quantity you need, and a photograph of the old needle and its packaging if you have either. If you have the system designation, that alone is usually sufficient. We confirm fitment before the order goes in, and we will say so plainly if we are not certain.