How to use an external variable in linkage section in COBOL and pass values from it into a new module and write into my new output file

Question

Could someone please tell me why a variable is declared as "External" in a module and how to use that in other modules through Linkage section and how to pass them into new fields so I can write it to a new file.

Answer 1

EXTERNAL items are commonly found in WORKING-STORAGE. These are normally not passed from one program to another via CALL and LINKAGE but shared directly via the COBOL runtime.
Declaring an item as EXTERNAL behaves like "runtime named global storage", you assign a name and a length to a global piece of memory and can access it anywhere in the same runtime unit (no direct CALL needed), even in cases like the following:

 MAIN
   -> CALL B
      B: somevar EXTERNAL
      -> MOVE 'TEST' TO somevar
   -> CANCEL B
   -> CALL C
      C: somevar EXTERNAL -> now contains 'TEST'

Answer 2

On an IBM Z mainframe, running z/OS, the runtime routines for all High Level Languages (HLLs) is called Language Environment (LE). Decades ago, each HLL had its own runtime and this caused some problems when they were mixed into the same run unit; starting in the early 1990s IBM switched all HLLs to LE for their runtime.

LE has the concept of an enclave. Part of the text at that link says an enclave is the equivalent of a run unit in COBOL.

Your question is tagged CICS, and sometimes behavior is different when running in that environment. Quoting from that link...

Under CICS the execution of a CICS LINK command creates what Language Environment calls a Child Enclave. A new environment is initialized and the child enclave gets its runtime options. These runtime options are independent of those options that existed in the creating enclave.

[...]

Something similar happens when a CICS XCTL command is executed. In this case we do not get a child enclave, but the existing enclave is terminated and then reinitialized with the runtime options determined for the new program. The same performance considerations apply.

So, as @SimonSobich noted, if you use CALLs to invoke your subroutines when running in CICS, EXTERNAL data is global to the run unit. But, if you use EXEC CICS XCTL to invoke your subroutines, you may see different behavior and have to design your application differently.