The or logical operator indicates that only one of the components of a logical expression need be true for the expression to evaluate as true.
exp or exp
The expression tax = 0 or value > 0 is true when either tax equals 0 and/or value is greater than 0.
if tax = 0 or value > 0 then...
The expression counter > max or value > 10 or y < 100 requires that only one or more logical expressions need be true for the complete expression to be true.
if counter > max or value > 10 or y < 100 then...
You can use the single attribute specification form of the or logical operator to optimize your queries. For example, when a numeric index on date is present, the expression:
select invoice with date = "jan 01 2013" or = "jan 02 2013"  16461 items selected.
should run significantly faster than the multiple attribute specification form of the expression:
select invoice with date = "jan 01 2013" or with date = "jan 02 2013"  16461 items selected out of 2206402 items.
Note that in the above two examples, the message  indicates that the search used an index and the message  indicates that no index was used.