Sales reservation takes place automatically through availability check
The setting is on the checking group ( OVZ2 - Define Checking Groups ).
You must set the "Accumul." column. Without setting this it will only check availability but not reserve it!
SAP recommend Accumul. = 3 -> Accumulate the requirements quantity when creating and accumulate the confirmed quantity when making changes.
For manual reservation, you can used MB21, movement type 251 - Goods Issue for sales.
To activate the Sales Order number field, do the followings :-
Activate transaction OMBW
double click movement type 251
double click additional acct. assign.
Tick the required/optional button as per your requirement.
The user have to manage the maual reservation using MBVR.
Reserving material without sales order
In Order to reserve the specific materials for a particular customer, use Strategy 50 to plan your MRP with the materials getting reserved for customer when you make Ind/Collective requirement(1) in MRP4 view of Material Master
or you can create a manual reservation against that particular material and give the customer name in the Receipent Field so that you can easily identify the Material which belong to the Customer