I am not entirly sure myself as they have just added many new keywords that are not documented.
Last time I checked struct copying was not possible (although nearly every C compiler supports it). Also last time I used it arrays can only be declared at the top of functions (like defined in very old C standards). There is no implicit type casting unlike C so even from byte to int will need a type cast opperation. I forget if C typecasting was supported but I suspect only native typecast functions are. It does support the entire compliment of bitwise opperators (|, ^, !, &, <<, >>) which allows bitwise manipulation. I do not think it supports bit fields in structs but these are not very useful.
It also declares arrays in a funny way.
type name[array size]; // c
type[array size] name; // galaxy
If you try using C features that are not supported the on-save compiler will usually give you errors explaining what is not supported (with specific reference to C).