DIM Service Format Description :

The format description parameter specifies the contents of a structure in the form T[:N][;T:N]*[;T] where T is the item type: (I)nteger, (C)haracter, etc. and N is the number of such items. The type alone at the end means all following items are of the same type.
The list of possible types (and their intrinsic data sizes) is:

"I" integer 32 bits  
"C" character 8 bits  
"L" long 32 bits (deprecated)
"S" short 16 bits  
"F" float 32 bits  
"D" double 64 bits  
"X" extra long 64 bits  

Example: "I:3;F:2;C" means 3 Integers, 2 Floats and Characters until the end.
(The format parameter is used for communicating between different platforms)