If you have a KITAS or temporary stay permit, to stay in Indonesia, you need to get a re-entry permit before you leave the country. If you don’t have one and leave the country, you could potentially have to start the whole application process for a new KITAS from the beginning.
I would suggest applying for a re-entry/exit permit or MERP, when you first get your KITAS or whenever you have to extend it. The multiple entry permit is stamped into your passport and you can leave and enter the country as many times as you like. You apply for the permit at your local immigration office. The single entry permit has been discontinued.
It takes 2-3 days to get the permit, but you should leave yourself plenty of time to apply, if you have an upcoming overseas trip planned. A 12 month permit costs 1,000,000 rupiah.
I would also like to note that a social visa is single entry. If you leave the country when you are on a social visa, you will need to apply for a new one to enter again.