In Finish to Start relationship, Primavera calculate Early and Late date base on 2 process
- Forward Pass (to calculate Early Start and Early Finish)
- Backward Pass (to calculate Late Start and Late Finish)
In Backward Pass, the Late Start of successor (Activity C) will be the base to calculate Late Finish of predecessor (Activity A and B)
How about in Start to Start relationship.
In follow picture, the relationship between Activity B and Activity C is Start to Start.
There is no activity standing behind activity B.
So how can P6 calculate Late Finish for activity B.
In Start to Start relationship Primavera will do it in reverse direction. It will calculate Late Start first, then Late Finish = Late Start + Duration.
First Late Start of B = Late Start of C = 25/Sep
Then Late Finish of B = Late Start of B + Duration of B = 25/Sep +5 = 29/Sep
Bonus :
In Finish to Finish relationship, the finish dates will be calculated first.
- In the forward pass, EF = predecessor’s EF
- In the backward pass, LF = successor’s LF
Then, the start dates get calculated from finish dates.
Khuong Do Blog 
The calculation procedure of P6 is to show a late start for an activity with a successor of only start to start relationship. Otherwise such an activity has, actually, a late finish date that is the project completion. The question is to what level such a relationship is realistic? We need to be careful with this open-finish relationship. In most of the cases the use of this relationship reflects improper planning or logic.
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