The tablet is a grid of four rows of five letters, read as EXARP, HCOMA, NANTA and BITOM. It was received during the scrying sessions John Dee conducted with Edward Kelley in the mid 1580s, in the same period as the great tables and the calls, and it survives in Dee's own manuscript record of those sessions, now among the Sloane manuscripts in the British Library. Whatever anyone concludes about where the material came from, the document itself is a sixteenth century artefact and its text is not in doubt.
Its stated function is structural rather than independent. The system Dee recorded is built around four great tables, one for each element, each divided into quarters and populated with names extracted from the letters by prescribed rules. The Tablet of Union is what holds those four together: each of its rows is assigned to one of the elements, and its letters supply the names governing the whole arrangement, so that four separate tables become one system.
In some of Dee's arrangements it sits at the centre of a cross formed by the four tables, which is where the alternative name black cross comes from, though the term is used inconsistently in the sources and in the later literature that depends on them.