To create a flag:

You can remove a flag by appending the character to command. Most commands accept - as argument-prefix as an indication to delete something.

Here there are some command examples:

[0x4A13B8C0]> fs symbols ; select only flags in symbols flagspace
[0x4A13B8C0]> f myflag   ; create a new flag called 'myflag'
[0x4A13B8C0]> f-myflag  ; delete the flag called 'myflag'

You can rename flags with fr.

Flag Zones

radare2 offers flag zones, which lets you label different offsets on the scrollbar, for making it easier to navigate through large binaries. You can set a flag zone on the current seek using:

Set scr.scrollbar=1 and go to the Visual mode, to see your flag zone appear on the scrollbar on the right end of the window.