SAP GUI: operational system environment variables

First kindly notice that variables listed below are valid only for WINDOWS operational system. Some of them are very useful (e.g. to set path to ini file). On other hand some of them are not very known (e.g. RFC related ones). The list is not complete for sure; anyways here it is:

1. SAPLOGON_INI_FILE – location of initialization file (saplogon.ini) with all the entries with connection strings to SAP systems
2. RFC_TRACE_DIR – location where trace files (rfc*.trc) are stored
3. SAP_CODEPAGE – to set code page for communication between GUI and SAP system (e.g. UNICODE systems). Used in cased when client has different code page (by default) then server.
4. RFC_TRACE_TIME – runtime analysis on the external RFC communication
5. DPTRACE – activating trace can be be activated using this variable
6. ABTR_TRACE – trace between GUI and RFC; trace is written when this variable is set
7. TDW_TIMEOUT – specifies timeout of connection to message server by default it is 10 seconds. 
8. SNC_LIB - variable specifies the path to the gssntlm.dll file.

P.S: in order to set operation system environment variable in WINDOWS use set command. E.g.: SET SAPLOGON_INI_FILE=%APPDATA%\SAP\SAPLOGON.INI