Within the port of Singapore, the MPA categorizes pleasure crafts boat regulations as either having a port license (SZ or SZH) or being a visiting yacht, vessels can be both but the SZ/SZH will supersede the flag. The common confusion comes as the Singapore flag registry has nothing to do with the Singapore port license, the Singapore flagged vessel is considered a visiting vessel similar to Langkawi, Cooks Island, etc.

Privately used pleasure craft in Singapore port waters, can not leave port to another country without a flag

Commercial charter pleasure craft in Singapore port waters, not to be used for individual fare-paying passengers.

(Including Singapore “SRS” Flag) Foreign registered to avoid port restrictions such as tax and crew.
The other type of prefixes you will see in Singapore are for harbor crafts, the prefixes at the bow will be SB, SC, SP, ST, and SR. All vessels will carry a different type of category e.g. SP is an individual fare-paying passenger like a ferry
If a pleasure craft doesn’t have a prefix or is different from the above they will be a visiting yacht that requires a cruising permit as below.