This means that:
The deductor is responsible for assigning a unique identification number (UIN) for each declaration they receive.
Each Form 15G / 15H should be assigned a UIN.
While filing the TDS Return, if you have deducted tax at lower rates due to Form 15G / 15H (Reason code 'B'), it is mandatory to declare the UIN in the deductee record.
The deductor is supposed to retain the UIN number and the forms submitted by the deductee for seven years.
This rule is only applicable for Non Salary TDS Return (Form 26Q).
The format of the UIN is as follows:
You should make sure to quote the same UIN if there were multiple deductions for the same deductee.