Certain effects can place cards onto the field face down (裏向き uramuki). Charms, Traps, and Seeds are all face down cards on the field, and the effects of certain other cards like Robin Hood, Jade General Queen can turn SIGNI face down.
Face-down cards are not considered to be SIGNI. They are not affected by effects that affect SIGNI, and you may put SIGNI onto the field onto the same SIGNI Zone as a face-down card. If a SIGNI with cards attached to it is turned face down, all the cards that were attached to it are put into the trash (or the LRIG trash in the case of Souls).
If a card that is visible to all players is turned face down without leaving the field, any player may check its front face at any time. However, if a card was placed from a hidden zone onto the field face down, such as through being placed as a Trap or Seed, only players allowed to look at that card by the rules or by other effects can look at them (for example, the player who placed a Trap or Seed can look at its front face at any time due to being allowed to by the rules for Traps and Seeds).
Face-down cards can be turned face up through certain card effects (or, if it is a Trap, by manual activation). Turning a face-down card face up does not count as it entering the field, so its 
abilities do not trigger.
Resonas, including double-faced Resonas, can be turned face down like any other SIGNI. A double-faced Resona does not become the SIGNI on its other side, but is treated as a card with no information like any other face-down card.
