A staff reserves a baseline minimum space above and below itself even with no extra content

Even a bare staff reserves roughly one staff-space above its top line and below its bottom line before any adjacent element may begin — engraving's floor, not its ceiling; every rule above ADDS to this floor when content requires more.

A system's top staff's uppermost ledger-line note sits closer than 1sp above the top staff line with nothing between them.

Ross, The Art of Music Engraving gives explicit baseline margin figures for a given staff size; Gould, Behind Bars states the general principle that staves are never packed edge-to-edge even when empty.

This rule is about where marks land on the page, so it can only be judged after engraving. Collision and spacing faults live here.