MS Project is using 3 type of tasks

1. Fixed Unit
2. Fixed  Duration
3. Fixed Work (I call it efforts)

Now, for me I like to use equation to illustrate these three type simple

D=W/U  (D: Duration, W: Work, U stands for Units)

Now if we played with this equation it can be show in different ways

1. For Fixed Unit: U = W/D. In this case increasing the Work will result in increasing the Duration and visa verse
2. For Fixed Duration: D = W/U. In this case increasing the Work will result in increasing the Unit and visa verse
3. For Fixed Work: W = D * U. In this case increasing the Duration will result in decreasing the Unit and visa verse

This is very important when planning for your project tasks

Note: For Units,I mentioned in case 2 & 3 that you can increase or decrease the Units,  what I meant by increasing or decreasing it is either increase and decrease the Units for the resource or adding more resource, in both ways you are changing the Units

and remember that using PMIS incorrectly will lead to fatal failures

Best Regards

Bilal Okour