SETLOCAL

Quote from: http://ss64.com/nt/setlocal.html

SETLOCAL

Set options to control the visibility of environment variables in a batch file.

Syntax
      SETLOCAL

      SETLOCAL EnableDelayedExpansion

      SETLOCAL EnableExtensions

      SETLOCAL DisableExtensions

      SETLOCAL EnableExtensions EnableDelayedExpansion

Key
   EnableDelayedExpansion  Expand variables at execution time rather than at parse time.

   EnableExtensions        Attempt to enable Command extensions. 

   DisableExtensions       Attempt to disable Command extensions. 

SETLOCAL on it's own, usually at the start of a batch file, will begin localisation of Environment Variables.

Issuing a SETLOCAL command, the batch script will inherit all current variables from the master environment/session.

Issuing an ENDLOCAL command will restore any environment variables present before the SETLOCAL was issued.

If a batch script does not use SETLOCAL and ENDLOCAL then all variables will be Global, i.e. visible and modifiable by other scripts.

Although global variables are easy to work with they are not good practice - for example if you have several batch scripts dealing with filenames (and these scripts may be CALLing one another), the first script may have a variable called _filename, the second script a different variable called file-name (a different name to avoid conflicting with the first script) a third script now needs something like file_name this quickly becomes very difficult to manage.

With local variables you are free to use the same variable names in multiple batch scripts - there is no conflict because the local variables are not visible to any other script. 
Local Variables can be passed from one batch routine to another with the ENDLOCAL command.

EnableDelayedExpansion

Setting EnabledDelayedExpansion will cause each variable to be expanded at execution time rather than at parse time.
EnableDelayedExpansion is Disabled by default.

Overloading a variable:

SETLOCAL can be used more than once in the same batch file so that multiple values can be stored in the same Environment Variable. To keep track of variable definitions, pair each SETLOCAL with a corresponding ENDLOCAL.

SETLOCAL is limited to 32 active instantiations per CALL level. At the root level a script can have up to 32 active SETLOCAL, and then CALL a subroutine that gets its own allocation of up to 32 SETLOCAL, etc.

@Echo off 
SETLOCAL
::Standard commission
Set _Commission=20 
Echo Standard commission %_Commission% 

::Premium commission
SETLOCAL 
Set _Commission=30
Echo Premium commission %_Commission% 

::back to Standard commission
ENDLOCAL
Echo %_Commission%

ENABLEEXTENSIONS / DISABLEEXTENSIONS

Command Extensions are enabled by default, there is rarely any need to disable them.

If Command Extensions are permanently disabled or if a script is running under the Windows 95 command processorcommand.com then SETLOCAL ENABLEEXTENSIONS will not be able to restore them. 

A batch file to warn if command extensions are not available:

   VERIFY errors 2>nul
   SETLOCAL ENABLEEXTENSIONS
   IF ERRORLEVEL 1 echo Unable to enable extensions

Errors

When run from a batch file, SETLOCAL will always set an ERRORLEVEL.
if given a valid argument or no arguments, the %errorlevel% will be set to zero, otherwise it will be set to 1.

SETLOCAL is an internal command.

"A local shop for local people" - The League Of Gentlemen