The use of items with charges is mandatory if it's conditions are met

The use of items with passive effects is mandatory

All other uses of items are optional

Item cards with circles to track multiple uses act exactly like persistent ability cards, in that if the situation applies, you must use the item in reaction and lose a charge. Additionally, it should be noted that if the card has a spent icon in the lower right corner, it is not technically spent until all charges have been lost, and so cannot be refreshed through resting or other abilities until all charges have been used.