You are looking at quite an old file. For portability, the original coding standard did not allow unqualified char types (without the signed or unsigned qualifier) because some compilers default char types to signed and some to unsigned. That has since been relaxed to allow unqualified char types if
the char is an ascii character or pointing to a string of ascii characters.
I’m also not sure I understand the full meaning of the parameter type “const signed char * const pcString”
That is a C question, easy to look up in any C reference book or on the web – we like to keep questions on the forum specific to FreeRTOS as it is more valuable to FreeRTOS users that way – there are plenty of general embedded programming forums [the answer in this case is that the type is a const pointer to a const string – neither the pointer, nor the string in points to, can be modified.]