Them of ALL rank character unit(2* to 6*) must be combine.
Because those cobined unit added Equipment frame.
This is two Equipment frames that is combined of two units,if there are three Equipment frames that three is.four is same process.
The new units(gacha,drop,etc) are that have only one a Equipment frame.
So later,It is mounted the New Equipment,It is that are four item,bracelets, necklaces, rings, but the name was forgotten consisting of another equipment.
Therefore,character unit of the Equipment frame desirable more often than is.
The details of which are, I think that wirte a coment of this wiki , blog , youtube and tells us maybe I asked to the Japanese.
http://bit.ly/2bnnyKk
http://bit.ly/2aYs6sF
http://bit.ly/2aHqg0t
http://bit.ly/2aQv0lF
http://bit.ly/2bc5Id7
http://bit.ly/2blrQCP
http://bit.ly/2b3tOYT
http://bit.ly/2bc6TZZ
http://bit.ly/2aMyQ9L
There knows.
By that, it would be useful for our play.
More easily, I think more quickly capture can be.