So indigenous to whatever region in which one finds oneself?

Here in Texas it means pecans. Spanish settlers named the Nueces River for the plentiful pecan trees growing along its banks. The river starts in the high reaches of the Texas Hill Country and flows down to Corpus Christi, in Nueces County, on the Gulf of Mexico.

It usually refers to walnuts

I've been looking for you, and I've seen it as a general term for nut, but mostly referring to walnuts or pecans in recipes. Hope this helps!