I am reading through the cost knowledge area of Rita 8th edition, but I am having a tough time understanding how to select the correct formula out of the 4 different ones.

The following are the formulas  of EAC: 

AC+Bottom up FTC

BAC/CPI

AC+(BAC-EV)

AC+ ((BAC_EV)/(CPIxSPI))

I have no problem in implementing them, but depending on the scenario (question) i am not sure which one will be used when...

Would someone be able to elaborate on this matter?  

Thanks guys!