The Adobe Type 1 Font Format book (PDF: 445 KB) - Adobe Partners

The entry StemSnapV is an array of up to 12 real numbers of the
most common widths (including the dominant width given in
the StdVW array) for vertical stems (measured horizontally).
These widths must be sorted in increasing order. For example,
you might include widths for straight and curved stems in upper
and lower case letters. For an italic font, this array should be
empty. For example:
/StemSnapV [85 102] def
If these stem hints are not present in the Private dictionary, then
each stem is rendered according to its own definition (as modified
by any other hints present in the font program).
The StdHW, StdVW, StemSnapH, and StemSnapV entries are relatively
new additions to the Type 1 hinting system. Currently
they are interpreted by ATM software version 1.2 (and later).
These features will also be recognized by future versions of the
PostScript interpreter.
5.10 ForceBold
At small sizes on low-resolution devices (such as display screens),
features of bold characters may be rendered at only 1 pixel of
thickness. Since this is the minimum thickness possible on a
raster output device, normal (non-bold) characters also appear
with 1-pixel wide features. If the boldness property is so important
at these small sizes that bold characters should continue to
appear thicker than normal characters, some Type 1 font interpreters
may apply special techniques to thicken bold character
features.
If the Private dictionary contains an entry named ForceBold, this
behavior can be controlled explicitly. The value associated with
ForceBold must be the Boolean value “true” or “false.” If the
value is “true,” then in situations where character stems would
normally be rendered at 1-pixel thick, a Type 1 font interpreter
may thicken the stem. If the value is “false,” then a Type 1 font
interpreter will not perform a special thickening operation. To set
ForceBold, use the statement:
/ForceBold true def
Adobe strongly advises font program developers to use ForceBold
to direct font interpreters as to which bold thickening behavior
is desired.
Chapter 5: Private Dictionary 43