What does the save button in a product's panel save?

It saves only what you changed, and its name tells you what that is. It reads Save cost, Save weight, Save packaging size or Save lead time. If you changed more than one thing, it reads Save changes.

A cost counts from its Effective from day onwards. Every day before keeps its old cost, as the message after saving reminds you. For a variant's first cost, orders before that day stay without a cost and are flagged. Pick an earlier day to cover them.

Under the cost fields, the panel shows the current cost and since when. It also shows how much your change adds or takes off per piece. Costs planned for later days are listed too. Saving from an earlier day replaces them, and a warning says so first. The ring at the bottom shows what the change does to your margin.

A weight or packaging size you save is yours from then on, and a sync never changes it. The days with that product are worked out again, so past parcels get the right price. Emptying the weight field does not delete the saved weight.

A lead time is a setting for the whole product, so it changes no number on your P&L. Only the reorder dates on Insights move.