Can someone help me Pls...

A little basic understanding is needed before understanding early and late dates which are calculated during the forward and backward pass during the CPM calculation.

If for an activity has budgeted units 100 (say), and at data date 'x' you have progressed 50% that means you have got 50 units which is for simplicity consider as actual units. Now the remaining units is equal to 100 minus 50 which is equal to 50. This 50 units would get distributed over the activity dates. P6 does calculates early dates during forward pass and late dates during backward pass. If the units are distributed using early dates then it is called remaining early units distribution and if distributed using late dates then it is called remaining late units distribution. 

I think overallocation comes into picture if the resource allocated to an activity is loaded excess than it's maximum capacity which is defined at 'max units/time' field.

Thanks Khaja Sharief.. your answer gave me better idea on Units Completion...