Energy Equivalence is an OBD rule created to deal with the effects of different systems of powerused by different universes. The following is Haterade's official definition:

Ability Overlap/Universe Equivalence:

This is where things become even more unclear.

I still see the odd new member say something along the lines of "that won't work because they have no reiatsu/reishi/chakra/ki/yoki/spiritual power/etc..."

To make it fair for all combatants, any ability that they have which is prevalent upon their opponent having the same type of power, that everyone in their universe has, is still usable. You may wonder "why is that?"

It's simple, not only does it give everyone a fighting chance, but it's also because all of the above mentioned forms of power are at least somewhat based on the same principle. Spirit.

Also, just because one character lives in a universe where there is no magic, that doesn't mean that all magical attacks from the other combatant aren't usable.

I admit that this one is veeery highly disagreed upon.

Notable Issues or Exceptions to the Power Overlap rule:

Next, it should be noted that any general universal ability that allows for an automatic win shouldn't be allowed, like Bleach character's invisibility due to them being spirits.

Another example is Kujaku's intangibility. Just because it makes him unable to be hurt, for the most part, it doesn't mean that he is able to withstand spiritual attacks. It also doesn't mean that his ultimate technique of causing all of his surrounding opponents to become intangible would work on someone with no physical body

There may be others that I haven't thought of at the time that this was posted.

In addition, if a combatant does not have a particular type of spiritual/energy power, then their strength/will is substituted. For example, we would assume that Luffy from One Piece would be able to break out of the binding spell that Rukia used onIchigo at the beginning of Bleach, even though he has no reiatsu, because his physical are feats way above that of Ichigo in the beginning of the series.

See Also:

General OBD Assumptions thread
Kamen Rider Ryoma's clarification of the concept