When you say "coding" what exactly do you mean? The GEDCOM spec is very specific and available on the internet for your review.
Now.. This being said... A "citation" as specified can be one of two types: 1) inline, 2) linked
An "inline" citation has all citation and source information imbedded within the (for example) EVEN.SOUR tag structure.
A "linked" citation contain only the citation information imbedded within EVEN.SOUR tag structure, and a link or pointer to a separate Source record.
If you are not familiar with GEDCOM this probably does not make sense. Note that each GEDCOM tag does not have any division within a specific tag. The specification requires a sub tag to have additional data points.
So. An example event based citation of type 2 would look like this:
1 EVEN
2 TYPE .....Type of event
2 SOUR .....link to Source record
3 PAGE ......WHERE_WITHIN_SOURCE
3 EVEN ......EVENT_TYPE_CITED_FROM
4 ROLE ......ROLE_IN_EVENT
3 DATA
4 DATE ......ENTRY_RECORDING_DATE
4 TEXT .......TEXT_FROM_SOURCE
5 [CONC|CONT] ......TEXT_FROM_SOURCE
3 ....MULTIMEDIA_LINK
3 ....NOTE_STRUCTURE
3 QUAY .....CERTAINTY_ASSESSMENT
The DATA tag has two sub tags DATE and TEXT. The DATA tag does not allow information specifically associated with the tag.
Hope this helps!