What does a partial data warning mean?

That a number on the page is computed from sources that are not all present. The commonest case is marketing: if one ad platform is not connected, marketing costs are incomplete, so the margins below them would be flattered if the page pretended otherwise.

The product's rule is to say so rather than to guess. A partial state names what is missing and what it affects, which lets you decide whether the figure is good enough for the decision you are making right now.