To make Prf items, use the ITEM Lock.
Items with Lock set will be dedicated to the unit or class, whose unit is the lock.
Even if unit have a weapon lock, unit can not use items unless the weapon level is set.
For example, Eirika who does not have a spear level can not use a spear.
Therefore, even if adapt Eirika Lock to a spear, she can not use a spear and can not use it.
Conversely, even if adapt Eirika Lock to a unit that can not use the sword, unit can not use the sword, so unit can not use rapier.
By using this characteristic, one Prf can be shared by multiple people.
At the very least we can not use weapons and magic at the same time, so we can double Prf.
If you can use a unit exclusively for ax and dedicated spear, you can increase more.