The [BOXGROW] token is used in the exact same manner as the [BOXFIT] token.
The only difference is the intended use:
•BOXFIT is used to get a box that is expanded until it is exactly the right size for the contained text, not too small, and not too large.
•BOXGROW is used to get a box that is expanded, if needed, to fit the contained text, but where the box being larger than the contained text is an accepted possible outcome. That is to say, the BOXGROW token makes the box at least large enough.
In practice this means that, if there is no text overflow at the beginning of the processing of a BOXGROW token, then no warning message will be issued.