GET & SET Parameter

ABAP/4 memory 

The contents of the ABAP/4 memory are retained only during the lifetime of an external session (see also Organization of Modularization Units). You can retain or pass data across internal sessions. The EXPORT TO MEMORY and IMPORT FROM MEMORY statements allow you to write data to, or read data from, the ABAP memory.

ABAP memory - Transfer data from program to program.
SAP Memory - transfer data from program to Transaction.

When you see 'Export and Import ' and it is ABAP memory'

When you see Set parameter id and get Parameter id and it is called SAP Memory

1. SAP Memory is a global memory area which all sessions within a
   SAPgui have access to. This allows for passing data between
   sessions. The SET PARAMETER ID and GET PARAMETER ID statements are
   used to manipulate the SAP Memory.
   
   SET PARAMETER ID 'MAT' field p_matnr.
   GET PARAMETER ID 'MAT' field p_matnr.
    
2. ABAP Memory is a memory area, which all programs in the call stack within the same internal session can access. The EXPORT and IMPORT
   statements are used here.
   
   export p_matnr = p_matnr to memory id 'ZTESTMAT'.
   import p_matnr = p_matnr from memory id 'ZTESTMAT'. 
3. Example : Parameter id allows you to store and retrieve values for a specific user, see below for syntax on how to perform these operations.
   The program displays a report containing what the value of the parameter id is before you run the SAP program and what it is after wards. Before will contain blank or what ever you entered last time the program was executed, and the after value will be the value you entered onto the ABAP selection screen this time.

   *Code to demonstrate how to set and get a parameter ID 
   *&-------------------------------------------------------------*
   *& Report  PARAMID                                             *
   *&                                                             *
   *&-------------------------------------------------------------*
   *&                                                             *
   *&                                                             *
   *&-------------------------------------------------------------*
   REPORT  PARAMID                                                 .
   
   data: gd_valuein(10) type c,
         gd_valueout(10) type c.
   
   PARAMETERS: p_value(10) type c.
   
   
   ********************
   *start-of-selection.
   start-of-selection.
   get parameter ID 'ZMESS' field gd_valuein.
   set parameter ID 'ZMESS' field p_value.
   get parameter ID 'ZMESS' field gd_valueout.
   
   
   ******************
   *end-of-selection.
   end-of-selection.
   write:/ 'value at start of program:',  gd_valuein,
         / 'value at end of program:',  gd_valueout.