Unit Quantity Code is a unique code for the measurement unit of a product. Eg: 50 Meters of cloth would be denoted by 50 MTR. When GSTR-1 is filed, HSN Summary must be reported with details of goods and services supplied during the return period.
The seller can use UQC while issuing a tax invoice, export invoice, credit note or debit note. The GST Portal accepts the codes from the UQC List only.
You can refer to the list of Unit Quantity Codes here - List of Unit Quantity Codes in India