1.
Thereon means on the object or surface just mentioned. (FORMAL)
There was a card on each door with a guest’s name inscribed ~.
ADV: ADV after v
2.
Thereon can be used to refer back to a thing that has previously been mentioned to show that the word just used relates to that thing. (FORMAL)
You will, in addition, pay to the Bank any losses, costs, expenses or legal fees (including VAT ~).
ADV: n ADV, ADV after v