This is Naval Jack of Thailand,Triranga defaced with Royal Thai Navy emblem, used since 2017. The RTN's emblem features anchor inserted in center hole of left-rolling Chakra. Over the anchor, there is a Thai royal crown. The emblem is yellowish.

The use of this flag: it is flown by RTN unit without proper unit flag ("ธงชัยเฉลิมพล") given by king of Thailand and also flown on royal yaucht. Note that "ธงชัยเฉลิมพล"'s design is the same as this flag, but it is mounted on wooden staff with nail. Also, the staff of "ธงชัยเฉลิมพล" is highly decorated.
Own work
Tangmo