You can take unwanted avatars (Materials) and Synthesize them with another avatar (Base) to greatly increase its Item Bond. Using avatars with MAX Item Bond as Materials will result in an even larger increase for the Base.
Stars are needed to Synthesize, and any avatars used as Materials will be lost.
Auto Select will automatically choose for you N and N+ rarity items with an Item Bond of 0 and ignore any items that you haven't maxed out, or that you only have one of.
What is Item Bond?
All avatars (except for face) have an Item Bond level. The upper limit varies by category. Item Bonds can be strengthened through Chatting and Synthesis.
When an avatar reaches max Item Bond, a golden crown will be added to its thumbnail icon.
This crown will also appear on items that you have previously raised to max Item Bond (excluding color differences).