Whenever something goes around something else, either literally or figuratively, partially or completely, this glyph marks the beginning of the area.
Mahadevan's sign number 287 probably shares a common root with the Chinese character bāo 勹, meaning to envelop or to wrap.
Illustrative Text References:
Harappa: Seal: H-1077 a: Asko Parpola, B. M. Pande, and Petteri Koskikallio, 2010: Corpus of Indus Seals and Inscriptions: Volume 3,1: Page 148: New material, untraced objects, and collections outside India and Pakistan: Suomalainen Tiedeakatemia.
Mohenjo-daro: Seal: M-168 a: Jagat Pati Joshi and Asko Parpola, 1987: Corpus of Indus Seals and Inscriptions: Volume 1: Page 50: Collections in India: Suomalainen Tiedeakatemia.
Image Credit:
Indus Script Sign Number 287: Sign List of the Indus Script: Iravatham Mahadevan, 1977: The Indus Script: Texts, Concordance and Tables: The Director General Archaeological Survey of India.