install/codepage/appl_server is a SAP Parameter attribute which is used to control codepages installed on the application server information. This is available within R/3 SAP systems depending on the version and release level.
Below is the standard documentation available and a few details of the attributes values .
Specifies the character set used by the system to store data.
Replaces as of Release 2.0 the system character set
specification in table TSYST.
Caution
Do not modify character set specifications without
consulting with SAP.
Changing the character set may require a character set conversion
of the data base.
See SAP Parameter Documentation for full SAP documentation for this profile parameter.