If x has more than one value (e.g. a vector of hex characters) they will be collapsed into a single value. If x is a list, then this is done to each list entry in turn.
For hexadecimals, i.e. when frombase is set to 16, any leading "0x" will be stripped for you.
The standard alpha "encoding" for digits in bases 11 thru 36 is assumed, i.e. 0-9,a-z .
Keep in mind that builtin functions such as strtoi will be much faster for base conversions supported by those functions.