A flat chest is exactly what it says, i.e. no breasts whatsoever. In other words, there isn't enough breast to cast even a shadow.

Lower Bound: None
Upper Bound: The chest cast a shadow

NSFW Example 1
NSFW Example 2
NSFW Example 3

Side Chart: In construction
Frontal Chart: In construction

IMPORTANT RULES: Characters breast should be evaluated with character in the standing position of the normal portrait picture (possibly a naked one) since close up and strange camera point of view can enlarge or reduce breast size perspective.
In case of borderline cases, always prefer to tag the character with the bigger size.
In case of enlarged breasts due to grotesque breasts/body modification just use:Grotesque Body Modification