What was Real De Asientos For In Aguascalientes During Colonial Times?

I am curious about what was Real De Asientos for in Aguascalientes and if there were those places in other towns of Mexico?

Also what is a Mercader?

I was going through the archives of Aguascalientes:

http://sigue.aguascalientes.gob.mx/axweb/

to get in username: sigue.publica
password: publica

Then I went to "Tipo Documental" and a long list came up and was looking at some. I looked at "compraventa esclavos" and it had a list of people that bought slaves, gives names, and the slaves they bought.

I was browsing through the names and saw that in 1724, "Melchor De Richarte Mercader, Fernando De La Campa y Coz Coronel De La Infantria Espanola" :

http://oi57.tinypic.com/hu0voh.jpg
http://oi61.tinypic.com/34f0go5.jpg

Here are the documents which is three pages:

http://oi57.tinypic.com/2lm3tdu.jpg
http://oi59.tinypic.com/2059zch.jpg
http://oi59.tinypic.com/29aw9jt.jpg

It is document (hoja): 1F-2F