The label element describes the text accompanying a glyph.
The text attribute is mandatory.
Its position can be specified by a bbox (optional).
Tools are free to display the text in any style (font, font-size, etc.)
The bbox element of a label is optional.
When no bbox is defined, the bbox of the parent glyph is
inherited.
The label should be drawn centered horizontally and vertically in the
bbox.
When the bbox is inherited, the label can freely spill outside (just
like it can spill outside its parent glyph).
An explicit bbox provides a stronger hint regarding what surface the
label should cover. It defines an upper boundary outside of which the
label should (ideally) not spill. It also represents a preferred size:
the surface covered by the label can be smaller, but should ideally be
as close as possible to the bbox.
In most glyphs (EPNs, unit of information, etc.), the label is supposed
to be centered, so the bbox is usually omitted (unless there's a
specific hint to be shared concerning the area the label should ideally
cover).
However, labels can be drawn anywhere inside compartments or complex, so
these should preferably have an explicit bbox.