Sleeve, the Steward of the Armed Shaped Empty Space

I am Sleeve, steward of empty space. I appear to surround, to define, to encapsulate an area, into which an arm may slide easily. "What I am is what I am, are you what you are or what?" Am I then defined by the space I appear to mark, by the space within my structure, or by the muscled, skin-strecthed appendage that will enter the folds? I am the sleeve, into which the arm will enter. I represent faith.