Which instance's timestamp is used as SCN in Oracle 12c rack? Let us assume 4 oracle12c rack instance's are running on different nodes. Each node has its own clock. No two clock shows the same time. 

Which instance's timestamp is used by Oracle as SCN?
And also upon how linearization is achieved.
SCN actually has a 3 second granularity now. As far as RAC goes, all cluster nodes must have synchronized clocks using NTP or some such; if the system times get too out of sync the cluster will start evicting nodes, trying to get them on the same page again. For more details on what SCN is and how it works, check these links:


http://www.dba-oracle.com/t_rac_scn.html
https://asktom.oracle.com/pls/asktom/f?p=100:11:0::::P11_QUESTION_ID:48052838748707
https://www.red-gate.com/simple-talk/sql/oracle/oracle-system-change-number-an-introduction/