What do the I's stand for in attenuation and power loss equations?

[calculator20]

I'm revising attenuation and power loss logarithmic equations.

The exam formula I'm given is attenuation = 10 log (I1/I2)

I'm trying to confirm what each I stands for.

Some of the examples I'm looking at seen to give input/output power, some seem to reverse it. Which is right? What do they stand for?

 

[BvU]

http://en.wikipedia.org/wiki/Decibel