thereof and thereon

I am reading the following...
The statutory definition of a real estate salesperson is any person, or business entity of not more than two persons unless related by blook or marriage,, who for compensation or valuable consideration is employed either directly or indirectly by, or afiliated as an independent contractor with, a real estate broker to sell or offer to sell, or to buy or offer to buy, or to negotiate the purchase, sale, or exchange of real estate, or to lease, rent or offer for rent any real estate or to negotiate leases thereof, or of the improvement thereon.
In the above paragraph, what do thereof and thereon refer to? I have looked them in the dictionary and there-of means of that or it, and there-on is described as on or upon that. My question is what is "that"? and is there a way to know exactly what it refers to or it is concluded from the context?
Thanks