[gentoo-user] MASKED Tilde with no `arch' what does it mean

[Harry Putnam]

Trying to emerge emacs-24, testing with:

emerge -vp =emacs-vcs-24.0.9999

I'm told its masked in /usr/portage/profiles/package.mask

But when I look there, I see:
  # Ulrich Mueller <ulm@g.o> (10 Mar 2010)
  # Emacs live ebuilds. Use at your own risk.
  ~app-editors/emacs-vcs-23.1.9999
  ~app-editors/emacs-vcs-24.0.9999
  ~virtual/emacs-24

With no explanation of what the Tilde means.

Looking thru man emerge in the MASKED section there is only one mention of
tilde and its in relation to notating architecture like ~x86.

All Ulrich M. says is `use at your own risk'

Ok, fine but first I want to find out what the unexplained tilde
means. There are only a handfull of packagew with that notation in the
masked file... where do look to find out its meaning?

PS- and by the way, there is no mention what so ever of `tilde' in 
man portage.