A CHAIN verb performs these steps:
1. Any USING parameters are first copied to a safe place. These are made available to the chained program through the CHAINING phrase of the Procedure Division header.
2. All active programs are made inactive.
3. A CANCEL ALL is implicitly executed.
4. A new program is loaded using the same rules that apply to the CALL verb. This becomes the main program of the new run unit.
For more information see the entry for "CHAIN Statement" in Reference Manual section 6.6.