Function keys are special keys on the keyboard (usually referred to as "Fn"). You can use them to access many functions in the R/3 System.
The R/3 System currently reserves 24 function keys. Each function key belongs to one of the following four groups:
Group 1: SAP Standard Function Keys
This group contains function keys which have the same function throughout the R/3 System and which should be permanently available. Even if a function of this group is not active in a particular task, the corresponding function key must never be assigned to another function.
Group 2: Mandatory Function Keys, if the Function is Active
This group contains function keys which should only be assigned to the functions specified for this group. Functions of this group do not need to be continuously active, however. If such a function is used at least once in a task, the defined function key has to be used for this function; otherwise, it can also be used for other functions.
Group 3: Recommended Function Keys, if the Function is Active
This group contains function keys which should be assigned to the function specified here (recommendation), if active. The key can be used for another active function as long as the recommended function is not active in a task.
Group 4: Function Keys with no Specific Function Assigned to them
This group contains function which are seldom used.
A number of functions must not to be assigned to any function key.
From Release 3.0c on you can define up to 99 virtual function keys (referred to as "Vn"). The assignment of virtual function keys to physical function keys is platform-specific. For functions which are included in a menu and are assigned a function key, the corresponding key combination is displayed in the menu.
Note that there are newer conventions for R/3 version 4.6 onwards. See Function Keys for details.
*) Will be included in the standard toolbar from Release 3.0C on.
Reserved Function Keys
Function keys V52 through V69 are reserved for the ABAP Development Workbench, function keys V87 through V99 for future (system) extensions (Note: V90-V94 are already used from Release 3.0d on).
Function Keys No Longer Predefined
The Select function is no longer predefined. If required, please use function key V9.
New Virtual Function Keys
If new virtual function keys are to be added, use function keys V25 through V48 as well as V21-V24.
Source: SAP Reference Lists