Ring of Force Shield bonus type

[zimmerwald1915]

Reading the description of the Ring of Force Shield, it does not say what type of bonus to AC it provides. Since it is wielded by the ring's wearer as if it were a heavy shield, and since it provides the same numerical bonus as a heavy shield, it would be logical to assume it provides a shield bonus to AC. This is not, however, specified in the item's description, and it could be reckoned that the bonus to AC is untyped on that basis.

Any clarification on this point would be appreciated.

[Frankthedm]

The item is clear enough. It is a shield bonus.

An iron band, this simple ring generates a shield-sized (and shield-shaped) wall of force that stays with the ring and can be wielded by the wearer as if it were a heavy shield (+2 AC). This special creation has no armor check penalty or arcane spell failure chance since it is weightless and encumbrance-free. It can be activated and deactivated at will as a free action.

[Archaeik]

Ditto, but pointing out this means it doesn't stack with an actual shield. Also, as written it should be considered bad for monks or any class that loses features when wielding a shield.
However, it still qualifies for attacks that don't ignore force effects.

[concerro]

The bonus type is what determines if things stack or not. Shield bonuses don't stack. They overlap so you only get the better of two.