This rectangular shape represents a mat. A good analogy is a Japanese tatami mat. The mat size was standardised. The size of both outdoor sales pitches and indoor floor areas was defined in multiples of the standard mat size.
Illustrative Text Reference:
There is no extant version of this Indus sign.
Comment:
This is one of only two Indus signs in the dictionary for which there is no extant version in the Indus inscriptions. I have included this sign because of its importance as a section header. The shape is incorporated into every sign in the Mat section of the dictionary.
Image Credit:
Floor Area: Lynn Fawcett, 2017.