can't do substitution for BSEG field

Dear experts, are all the fields in the BSEG field be substitutable? is there any fields in BSEG we can't define FI line item substitution for them, how would I know the field can't be substitutable?

also for substitution, what's the difference between define substitution at line item and complete document level, if we define substitution at complete document level, does it asl affect document header? thanks