The letter in a fit designation that positions the tolerance zone relative to nominal size.
The letter is the second half of the story. Lower case is a shaft, upper case a hole. Letters a–h place the zone below nominal (clearance), j–n straddle it (transition), and p–zc place it above (interference).
H and h are the special cases: an H hole is never smaller than nominal and an h shaft never larger, which is why H and h are the basis of the two fit systems.
Reading the letter as part of the tolerance size. It sets position, not width.
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