In Chatsecure conversations, what is the meaning of the little ticks and crosses? I noticed that after each message when OTR is enabled, 2 little ticks or crosses appear in the message bubble. Sometimes they change color from black to white.

What is the meaning of these indicators?

I think it something to do with message delivery and /or to flag the recipient has read the message, but I would appreciate a definitive explanation!
One check means it was sent.

Two check means it was delivered, though not necessarily read.

One check with One X means it was sent, but no delivery receipt was received.