Use-activated magic items and determining their action cost

Question

I was reading the rules for Using Magic Items and there is a part which just isnt clear to me in how it works.

2nd last paragraph, just before size and magic items:

Unless stated otherwise, activating a use-activated magic item is either a standard action or not an action at all and does not provoke attacks of opportunity, unless the use involves performing an action that provokes an attack of opportunity in itself. If the use of the item takes time before a magical effect occurs, then use activation is a standard action. If the item's activation is subsumed in its use and takes no extra time use, activation is not an action at all.

When you need to activate a magic item, most tend to be a standard action, but some are not. How can you tell/know when the magical effect is suppose to occur? Yes most items to mention what their action cost is, but its when creating new ones that dont have that built in description.

Answer 1

Most things are supposed to occur on activation. Your wand should fire when you point, your magical mace grows spikes when you tell it to, your armour starts turning into bees when you command it to, your potion does its thing when you chug it, and so on.

Those that don't are anything that is "subsumed in its use", which is to say occurs during another action. Such items would include 'kicker' items that boost something else without being actively 'used' or magical weapon properties that occur upon hitting a target.

A good example here would be the Book Of Extended Summoning , which allows the user to apply an effect to a spell as they are casting it and therefore are just consumed as part of the spell, therefore not taking any action to engage.