If you are writing a custom function in Vimscript that operates on a range, you may not want to rely on the '<,'> range to get the visual selection because it can also refer the previous selection which you are no longer interested in.

To ensure that you are only operating on the current visual selection, mark your function as a range function by adding range after the argument list:

"Convert GitHub-Flavored Markdown syntax-highlighting to Liquid syntax-highlighting.
function! ConvertToLiquidHighlighting() range
  silent! execute a:firstline . "," . a:lastline . 's/^```\([a-z]\+\)$/{% highlight \1 %}/g'
  silent! execute a:firstline . "," . a:lastline . 's/^```$/{% endhighlight %}/g'
endfunction

"Convert within visual selection
vnoremap <leader>H :call ConvertToLiquidHighlighting()<cr>
"Convert entire file
nnoremap <leader>H  :0,$call ConvertToLiquidHighlighting()<cr>

With the range keyword in place, the a:firstline and a:lastline will be available within the function and will contain the line numbers of the current visual selection’s first line and the last line, respectively.

:help func-range for more info.