SKTT is a residence document printed by Catatan Sipil office (Registry Office).
This document confirm your address where are you staying which is provided with NIK number and also one of document needed for immigration to obtain your stay permit extension.

Law #23 year 2006 about Population Administration
Article 20
1. Foreigners who have a Stay Permit coming from abroad, and other licensed for Foreigners who have changed their status as holders of a Stay Permit, who plan to reside in the territory of the Republic of Indonesia shall report to the Kantor Catatan Sipil no later than 14 days from the issuing of the Stay Permit.
2. Based on the report, referred to in paragraph (1), the Kantor Catatan Sipil issues a Certificate of Residence.
3. The validity period of the Certificate of Residence as referred to in paragraph (2) shall be adjusted to the validity period of the Limited Stay Permit.
4. The certificate of Residence as referred to in paragraph (2) shall be carried at the time of travel.