What is used on [0]?

 
In functions that reference the bar, just what is used on [0]?
For example, what is Close[0]? I would expect it is the current price (bid?). However, my EA isn't behaving that way, so maybe it's something else?
Another example, the MA function iMA(.....,0) where that '0' refers to the current bar. If the MA is using closing price to calculate the value of the MA, what is it using for the current (0) bar? Close? What if it's using some other method?
Open, High, and Low seem to have clear definitions for a current bar, but Close just seems to be troublesome. To me it would be obvious to be current price (as if it were to close this instant) but my confidence in that assumption is threatened by bad EA behaviour.

Thanks for the help.
-matt
 
Another way to ask it,

Why would Close[0] be different from iClose(NULL,0,0)?
I haven't tracked it down, but I get different behavior from EA when I switched from former to latter.
 
Sample of code please to reproduce problem
Reason: