CommentStreams:413106dddbc8bf11d8a733cbf40d64cc

Correct, could only be set by guildmaster, and it's visible to all players in your guild. It lasts for one round, so if your guildmaster never sets it or sets it for the rounds when you are sleeping you'd never see in the battle field (but you would usually see it as an event in the guild log).

I personally set it 3-4 times per war to indicate the most important cells to capture. Also, sometimes when some important square has lots of enemy troops and I am going to send a lot of my troops there this round.

Many players simply ignore the flag, so some guilmasters may not even bother to set it.