After you redeem capitastar voucher in Grab app, you have to transfer the voucher to Capitastar App and spend it with Capitastar App!
Not all shops accept payment in Star$ in "loose points", many only accept min $5 evouchers or in multiples of $5. It is a smart app, it will always use up evouchers which are going to expire first.
It will also use evouchers which are expiring before the Star$ points first, ie based on expiry date. So my points still remain in the app unused. If the shop accept payments in Star$ in "loose points", it will deduct the points from the evouchers based on expiry date.
For eg, I won a $50 evoucher and it is expiring first, I spent some, now the balance is $46.91. My loose points are expiring after this evoucher, it stays so I can continue to accumulate and when enough, I can redeem voucher at 4500 pts.