Here is the text from Your Story, page 280:
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Crafting Specializations
Crafting specializations for items and potions aren’t used for control of complexity; they usually affect frequency or strength without making you spend an extra slot to do it. A frequency specialization allows you one more use per session. A strength specialization increases the effect strength of your basic enchanted items by 1 (this strength specialization bonus can’t be traded in for an additional per-session use). In the case of potions, the can create stronger potions, or ones that you can get two uses out of. Alternately, a crafting specialization may be applied to increase the limit on how many bonuses may be placed on a single focus item (a focus specialization).
You can create focus items which are used to provide frequency and strength bonuses for crafting when making other items and potions. That said, you can’t create a focus item that helps you create other focus items.
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Now, given these rules, my question is: if my wizard makes a potion or one-use magic item with a duration of (let us say) 3 exchanges, what does an added Frequency focus or specialization give? Does it give two identical uses of 3 exchanges, or does it give one use of 4 exchanges?