The docs for this is in `:help v_CTRL-A`.
It should actually say VISUAL BLOCK mode. This is where you use `CTRL-v` to select a block of text. You can read more in `:help blockwise-visual`.
Also see the 'nrformats' option: `:help 'nrformats'`. You can include alphabetic characters to do the same thing with A-Z, for example.