In MR21--> You have to enter the new price and it will direct change the moving average price or standard price with refer to the material master settings.
   
E.g. Material No: ZZZZZZZ
      Material Price: 20.00

If you use MR21 you can change the price directly in to 23 or 25 whatever you need.
   
In MR22--> You have to enter the amount needs to be adjusted and it will hit the moving average price or the standard price with refer to the quantity you are having in stock.
   
E.g Material No: ZZZZZZ
      Material Price: 0.17
      Quantity: 191 Nos
      Total Value : 31.57 ( 31.57/191=>0.17)
   
If you enter debit or credit to this material.

Assume if you want to add 100.00 to this material.
   
After the change ==>
     Material No: ZZZZZZ
     Material Price: 0.69
     Quantity: 191 Nos
     Total Value : 131.57 ( 131.57/191=>0.69)
   
Instead of increase the value you can decerease also. (by enter negative value)
   
This is the difference between this two transactions.

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SAP Material Management Interview Questions, Certification and Configuration Books

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