Short answer

The SM and M symbols identify stocks listed on special exchanges for small and medium enterprises:

You can view and trade these stocks directly on Arrow. However, you must trade them in multiples of their lot size, and you cannot buy a single share or any odd quantity.

NSE Emerge and BSE SME platforms

NSE Emerge (PDF) provides a platform for emerging orporates to raise capital from Institutional Investors and HNIs through a credible admission process. This exchange finances Small and Medium-sized enterprises (SMEs) and includes companies with high growth potential, subsidiary companies, companies funded by VC firms/angel investors, and project SVPs.

BSE SME (PDF) operates similarly, providing a platform for SMEs and investors to meet and transact.