Nonexistent. Consult your dictionary on the existence of the words hereto, herein and hereof. They are not mentioned or considered as (highly) ‘formal’ or ‘written’ language. Where a dictionary gives any examples, you will recognise them as contract ‘legalese’. English dictionaries also tend to trace the meanings of those words back to U.S.English. One of the manuals on contract drafting, albeit by a ‘contrarian’ author, advises that as “their distance from common speech makes them prime candidates for the chop, and they are usually easily dispensed with.” It explains why these words are not only archaic legalese but also vague.
Meaning. What does it mean? Typically, the here-part refers to the agreement (the document in which it is written). Accordingly, hereto means ‘to this agreement’. It is often used as an extension (e.g. “the parties hereto“) or to identify the placement of a schedule (“attached hereto as”). Hereof means ‘of this agreement’ (e.g. “as of the date hereof“).
Try to overcome the habit of using hereto, hereof and herein. Be specific and write out “to this agreement“: it is difficult to conceive that “the parties” refers to something else than “the parties to this agreement” (and if there is reason for doubt, define the parties as “Parties” in the party blocks):
Seller and Purchaser are collectively referred to as the Parties and also individually as a Party.