Rule Question: let's talk Save Re-rolls!

Hey everyone!

I'm wondering about save re-rolls, especially the ones the Mortek Guard have in their Shieldwall ability. "You can re-roll save rolls for attacks that target that unit until the end of that combat phase."

So - does this mean I can pick and choose the save rolls I want, or do I have to re-roll all of them, failed or not? I know some warscrolls have a similar ability worded as 're-roll FAILED save rolls'. But it's obvious how that one works.

I'm hoping someone could clarify this for me. Thanks a bunch r/AoS!