Question about variable definitions

Hi

I'm a VC++ programmer doing my first bit of embedded development in C for a 80C52 compatible chip. I have the use of some API's and included is an .h file containing the following typedefs:

typedef unsigned char data Ucd; typedef unsigned char idata Uci; typedef unsigned char pdata Ucp; typedef unsigned char xdata Ucx; typedef unsigned char code Ucr; typedef unsigned char Uc;

Code samples available to me use Ucr as a parameter type for a function if the data being passed is a "string" rather than a variable. I understand vaguely that different data may be stored in different memory spaces, but I guess I need to know the following:

1) Is there a good book, document, web-site, or what ever that will help me understand the basic concept of why these typedefs are needed and 2) Do I need to use these typedefs or can I simply use unsigned char? 3) If there is a choice as to whether or not these typedefs are required, what are the advantages/disadvantages, etc. of using them

Many thanks in advance for any help with this. Julia.