This tip explains Lag with Dependencies other than Finish to Start, like Finish to Finish and Start to Start, in Microsoft Project 2013.
First we create a simple project:
As you can see in the Predecessors field, Tasks 1 and 2 have a Finish To Finish (FF) relationship, and Tasks 3 and 4 have Start To Start (SS) Dependencies. Now, suppose that Task 2 and Task 4 have two days of lag:
Task 2 will have a finish date of August 4, 2015 and will start two days later, so its start date is postponed!
When we repeat this for Task 4 we get:
The same result! The point to remember is that, yes, you can have Lag (and Lead) in tasks with dependencies other than Finish to Start, but be aware of consequences!