Wat do we mean by the below syntax...
1. FILEDEF &BCGI_ID DISK /tmp/&BCGI_ID.txt
2. !mkdir /tmp/&BEXT_NAME

1. FILEDEF &BCGI_ID DISK /tmp/&BCGI_ID.txt
means 'refer to the file in application directory temp called &BCGI_ID.txt as &BCGI_ID', where whatever the value of &BCGI_ID will be used. This file could then be populated from, say:
ON TABLE SAVE AS &BCGI_ID

2.!mkdir /tmp/&BEXT_NAME
means make a directory called whatever the value of &BEXT_NAME is, under the application directory tmp.