In a buffer with "hybrid" line numbers (:set number relativenumber), numbertoggle switches to absolute line numbers (:set number norelativenumber) automatically when relative numbers don't make sense.

Relative numbers are used in a buffer that has focus, and is in normal mode, since that's where you move around. They're turned off when you switch out of Vim, switch to another split, or when you go into insert mode.

Using vim-plug

1. Add Plug 'jeffkreeftmeijer/vim-numbertoggle' to ~/.vimrc or ~/.config/nvim/init.vim
2. Run :PlugInstall
3. :set number relativenumber

Once help tags are generated, you can view the manual with :help numbertoggle.