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. 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.

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.

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, "@Abcd E F" will notify user "Abcd", if he made any comment previously on the same post, otherwise no one will be notified. "E F" will be considered as the body of comment. It's better to use @AbcdEF to notify the target user, here, "Abcd E F". In short, spaces must be neglected while writing the full name.

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. 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.