In insert mode, the functions of <c-j> and <cr> are also the same. However, when <cr> is mapped to <Esc>, when <c-j> is pressed, the normal mode will not be entered.
I feel that the relationship between <c-i> and <tab> is similar to the former, but the performance is different. I use <tab>Map emmet's <c-y>, but <c-i> will also trigger <c-y>. The same is true even with non-recursive mapping inoremap.
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Please learn to use :help [key position] to view their essence, for example
:help i_CTRL-Jis like this:Another example
:help i_CTRL_Iis this:I think the help documentation is pretty self-explanatory.