A digital signature is a way to authenticate the authorship of a file. Only OpenType and TrueType fonts are capable of being signed with a digital signature as the signature is stored in an OpenType Table named DSIG. In the case of TrueType flavored OT fonts, the digital signature is the singular key which determines whether or not the font is marked as an OpenType font with the O icon. This means that TT flavored OT fonts that are not signed will not display the O icon and TT fonts that are signed but have no other OT features will display the O icon.
Digital Signatures for Font Files.