Dear Fahad,
Accumulators are nothing but place holders used as basis for calculating benefits.
Suppose if any benefit is contributing to the calculation of other benefits, then accumulators will come in to picture.
Calculation orders are used to tell the PEP Engine to sequence the benefit calculation.
Suppose Benefit 1 is contributing to benefit 2, then benefit 1 should be calculated first.
Calculation Order 1 is the highest and will be calculated first, then 2, 3, 4 and so on...
There is no upper limit for calculation order 
. only thing is order 1 takes the highest priority.
Calculation Basis - BASE (Reserved Key Word) means Base Salary (Periodic Salary) is the basis of cacluation.
Configuring
Accumulators:
To configure Accumulators, one has to follow the following Path:
Step 1: PBF Administration-> PEP Maintenance-> Benefit Rules->Select the Benefit Area, Click on Next-> Select the required Benefit Plan, Click on Next-> Click on Edit-> Click on ADD to maintain
the Accumulators.
It means those benefits will be contributing to that particular Accumulator.
Step 2: In Benefit Rules, you have to set up that accumulator as a calculation basis for other benefit.
Example: There are two benefits HRA and SPL associated with Employee E1.
HRA benefit is contributing to the SPL. I mean SPL is 40% of (Salary + HRA benefit).
Then define a Accumulator GROSS (example). HRA should contribute to Accumulator GROSS. (Refer Step 1).
Then set the calulcation basis for SPL as GROSS in Step 2.
Also set Calculation Order for HRA as 1 and SPL as 2.
I hope I have answered your question. Kindly revert if any more clarification needed.
Thanks & Regards!!
Subramanya