This is for text composition software that toggles bold and italic on and off. If the user highlights a word of Times and clicks the "I" button to italicize it, the font is replaced by Times Italic. The "Font is Italic" indicator should be marked in Times Italic so that the program knows that clicking the "I" button can only switch italics off again by replacing Times Italic with Times. If that marker were missing in Times Italic, its place in the family would be broken, and italicizing it would make the program artificially oblique it further.

This system was not designed with families of more than two weights in mind. Some people group them into sets, but best practice is to mark "Font is Bold" for Bold only (and both for Bold Italic, of course), and leave all other weights to be selected by the user directly.

Oh yeah !
I understand this answer !
Thank you so much, @cerulean !