Just wondering why when i select several frames in an InDesign file i can then click on a frame within the selection and it becomes active (darker blue frame).
That indicates it's the "key frame" for alignment when you are aligning to selection.
That's the way you designate a "key" object for Align/Distribute/Spacing functions. Suppose you had 6 objects, and you wanted to align them all based on the current location of one. Select them all, click again on the one you want to remain stationary (while the others move to align to it) to make it the key object, then click the desired button in the Align panel.