As @JetSetIlly said above, that is a mirror of one of the collision registers.
As for the unused "Floating" $3E,$13E etc addresses above, it can vary console to console, and emulator to emulator, flash cart to flash cart. I have found them to read value 0x00 in my consoles. It is better practice to guard code against reading the addresses, even if every console behaves the same. Emulators in developer mode can help you find the situations by selecting the "drive pin to random" function, your program will behave erratic with that enabled. Most developers who will be running on developer mode and when they try out your game it will do weird things like roll the screen, drift sideways, have uneven scanlines, etc.