ASCII and Strings
In the AB universe, there are two types of "string" files: A (for ASCII) and ST (for String).
Each data type is used in different ASCII instructions.
There is absolutely no difference between an A file type and an N file type. A files may be used with any instruction that an N file type is used for. The only difference that I've been able to figure is that RSLogix open A files type with the ASCII radix instead of the decimal radix.
ST files are used with ACN (Concatenate), AEX (Extract), ASC (Search), ASR (Compare), ACI / AIC (String-to-Integer, Integer-to-String) and the Channel 0 instructions (ARD (Read Buffer), ARL (Read Line), AWA (Write Append), AWT (Write).
There is a slight difference between an ST file type and an N file type. Link a T or C type, the ST is structured. Whereas T and C consist of 3 words, a single ST element is 42 words long (regardless of the data in the string).
The ST is structured with the first Word is the length of the string. The next 41 words contain the (up to) 82 chararacters in the string, in ASCII format (the same as you get when you look at an N file using the ASCII radix).
You can convert one data type to the other:
COP #ST10:0 #A11:0 41
The Length part is first, so that will go into A11:0. The string will be A11:1 to A11:41.
MOV 41 A11:0
COP #A11:0 #ST10:0 1
Will convert it back. If the string that you want to store in the ST file is not 82 characters long, put replace the 41 with your string length (less than 41, or weird things happen) in A11:0.
Like Tom said, the "number" in the String is in ASCII. If you want it be a number, you need to convert it. The ACI instruction will do that.
The individual elements of the ST data type can also be referenced directly. The length is ST10:0.LEN. The first two charactors (since each charactor is one byte, and the PLC references everything by word) is ST10:0.DATA[0], the 3rd and 4th are ST10:0.DATA[1], etc.
Again there is no difference between ST10:0.DATA[0] and A11:1 - both are single word entities.
I think that more than answers your question.