The flag has a website with quite a lot of content: https://sussexflag.wordpress.com. Here you can read about the history and symbolism of the flag, and for example see the six rapal flags of Sussex:

Apparently it is driven by some enthusiast, highly engaged in the flag. My favourite part is the page cracking down on incorrectly used flags for Sussex, including images like those below. https://sussexflag.wordpress.com/councilflags/