I'm aware that symbol means other things. It was popluar symbol in a few different cultures but it's known for one thing in particular these days.“The symbol is known as Manji, and it's the "Ban" in "Bankai". "Bankai" is written as "卍解". Ichigo's Bankai tsuba is shaped like the first symbol of the word Bankai, that's all.”
quoted someone who knew the symbol. It’s basically a “letter” in the simplest Explanation.