How do we normally write PLSQL code that uses env variables?

IHAC (11i E-Business suite, but this is a rather generic question) that would like to know how Oracle accesses the values of the environment variables like AP_TOP, AR_TOP, APPLCSF etc. in PL/SQL code because they want to avoid hardcoding paths in PL/SQL blocks.  How does Oracle's standard code get the values that are assigned to environment variables like AP_TOP, AR_TOP, APPLCSF etc. in a PL/SQL block ?  They are currently using DBMS_SYSTEM.GET_ENV to achieve this, but for that they have to put the custom variables in custom$CONTEXT_NAME.env in the RDBMS ORACLE_HOME.