I am confused while addressing the user in comments.. if the user name for example "Abcd E F"
How do I address it, @Abcd E F or @AbcdEF ??
According to this answer on How do comment @replies work? - Meta Stack Exchange
Spaces are removed from the display names for matching purposes. So to match Peter Smith you may use @pet, @peter, @peters, or @petersmith. The last two are useful if Peter Jones is also participating, who can then be distinguished using @peterj. However, no spaces are allowed in the @name itself. Like to notify P Smith, one must use @psm or @psmith. (Here @P Smith would be handled as just @P, which is too short.) Single quotes, dots, dashes and underscores should not be removed.
So, any combination of greater than three alphabets starting with first name will notify user "Abcd E F", if he made any comment previously on the same post and if there is no conflicting username participating. It's better to use @AbcdEF to notify "Abcd E F", to avoid any kind of conflicts. In short, spaces can be neglected while writing the full name.