Referring to a buffer, and :help help

I've two files loaded, FileNumberOne in buffer 1, and FileNumberTwo in buffer 2, with just FileNumberOne currently shown. I want to open FileNumberTwo in a new split window. I can do ":split FileNumberTwo" to achieve this. However, I don't want to type out or use completion to refer to FileNumberTwo. Some commands will accept "#2", or even part of the buffer name as an argument, but not :split (and some others) - or can they?

I see the usage of split is ":[N]sp[lit] [++opt] [+cmd] [file]", so I was trying to find help on what "[file]" means and what exactly I can substitute it with, but don't know how to search the help for "[file]". What would I search for for that? And [++opt], [cmd], etc. Are they in the help somewhere?

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