Each record in the instance is identified with a unique 18-character ID called a sys_id.
Record ID must be unique within the same table (including records in parent/child tables).
Record ID value cannot be changed.
How to get a record ID
...
The record ID can be obtained in several ways, manually and automatically (via API).
Getting the sys_id from the record URL
The record sys_id is always a part of the URL that leads to this record; so you can retrieve the sys_id from the URL.
...
Has a sys_id equal to '158815469913225806'. It can be obtained either from a browser address bar or by copying the URL of the item at the navigation panel or on the list view.
Getting the sys_id from the script
The sys_id can be retrieved not only manually, but using scripting, too.
...