You can invest extra character points (CP) into any weapon that you have a Weapon Bond with. If the weapon is ever lost/sold/destroyed/etc, all of the points are lost as normal for Weapon Bond.
These points add magical effects to the weapon, and will persist on the weapon even if you no longer are its owner. If you transfer a Weapon Bond to a new weapon, the enchantment(s) stay on the original weapon.
When you spend your points, energy is added to your weapon. It doesn’t have to be allocated right away. But once you assign energy to an enchantment, it cannot be reallocated.
Ideally, each weapon will end up with a unique suite of enchantments that reflect its use or its wielder.
The effective rate is 75 energy per CP spent. Look in the Magic book for details on how much various enchantments cost. Unused energy in a weapon is not lost, and can be used for other enchantments in the future.
Example: Flaming Weapon
Flaming Weapon has an energy cost of 750. It will take 750/75 = 10 CP to give your bonded weapon +2 fire damage.
Example: Accuracy +1
Accuracy +1 has an energy cost of 250. It will take 250/75 = 3.333 = 4 CP to give your bonded weapon a +1 to hit. Also, since 4 CP were paid providing 300 energy, and only 250 energy was used, that leaves 50 energy waiting to apply to the next enhancement.
With various other spells throughout the Magic book, you can achieve almost any effect/ability that can be replicated with magic. If you have a grander idea in mind that the default stuff that is listed, let me know and we can figure it out.