TL;DR is that in .Bd -offset indent the "indent" is an... let's say an
enum value (with other predefined choices being "center", etc). For
lists, .Bl -offset indent is the only such predefined value (no
centered lists...)
.Bl -tag -width indent is a weird contamination (in the linguistic
sense), or a "confusible" to borrow a more colloquial term from the
eponymous "Room's Dictionary of Confusibles".
-width takes three kinds of arguments. Explicit nroff width
specification like "8n" (8 en-widths of the current font), "1i" (1
inch), etc. An mdoc macro name that implies mdoc's default choice for
"that kind of item", e.g. .Bl -tag -width Fl for items that are .It Fl
... Or you can provide an "example" - one that nroff will format and
compute the width of, and that computed value will be used.
! The rule of thumb is that if your items are .It Xx foobar for some
! macro Xx, the -width specification of the list should be either
! plain Xx to use the mdoc's default, or some specific invocation of
! Xx from one of your items. E.g.
.Bl -tag -width Dv
.It Dv FOO
.\" ... description of the macro (_D_efined _V_alue) FOO
...
or, say
.Bl -tag -width ".Dv LONGERMACRO"
.It Dv FOO
.\" ...
.It Dv LONGERMACRO
.\" ...
This latter form provides space wide enough so that the example tag
value from .Bl can fit in that space without a line break.
The very common mistake in the above is
.Bl -tag -width LONGERMACRO
.It Dv LONGERMACRO
that is invisible in text manpages, as ".Dv LONGERMACRO" and
"LONGERMACRO" has equal width in plain text. But in PS the .Dv uses
wider fixed-width (Courier) so the width of "LONGERMACRO" in the
proportional body-text font (Times) is not enough to fit in the actual
item.
(My current preference is that this latter by-example form is to be
avoided to get consistent indentation even if the typography may be a
bit awkward).
Anyway, -width indent is really that kind of by-example indentation
that asks mdoc to set the word "indent" in the current font and use
its width. It's not a predefined "enum" value like "indent" in
-offset
Hope that clarifies it.