"In order to find what change occured at the number 6, what exact command should be used?" - you'll need to compare it with the previous configuration that was present (e.g. was committed during the previous commit). In your case, it will be commit #7, and the correct answer is "show system rollback compare 7 6".
This example should make it clear - let's see what was committed during the last commit (#0):
user@mx# set interfaces lo0.0 description test1
user@mx# commit and-quit
Exiting configuration mode
user@mx> show system commit
0 2019-10-10 09:48:28 CEST by user via cli
1 2019-10-08 11:32:30 CEST by root via other
user@mx> show system rollback compare 1 0
[edit interfaces lo0 unit 0]
+ description test1;
Can you please also accept one of the posts as the solution, if now everythign is clear for you? It will mark this question as answered and help other community users find the solution faster should they have the same query.