clear and reset both opcodes are used for clearing the 
fields.difference is that clear only clear but reset is 
used for clear  & then initialise the numeric fields with 
zero and character with blanks.

 OPEN & CLOSE opcodes are used for explicitly open and 
close the file.In precise all files are automatically open 
when these are processed by program bt at somepoint if we 
want this should not be implicitly then open we use these 
keywords.    
 SFLNXTCHG :Its always read those records which changed 2 
or more times.this is declared in subfile(SFL) format not 
SUBCTL format.

clear and reset both opcodes are used for clearing the 
fields and then initialise the numeric fields with 
zero and character with blanks difference is that if we 
define any initilialise value for that var in pgm reset 
initialize it by that value but clear initialize it by zero 
or blanks.
 OPEN & CLOSE opcodes are used for explicitly open and 
close the file. 
    
 SFLNXTCHG :Its always read those records which changed  
in uppdatesubfile.this is declared in subfile(SFL) format 
not 
SUBCTL format.