Hello John,
Reference field is belongs to Header level data of a document. Document header level data is stored in BKPF table. You can find all the header level information of the document in that table.
Also if you want to know the details of table which it got stored just click F1 key and then press technical information i con in order to know the table details in which data is getting stored. I am agreeing with Dev's reply.
Thanks & Regads,
Lakshmi S