What are the criteria for distinguishing between a deductible expense and a capital expenditure?

The criteria for distinguishing between whether an expenditure is a capital item or a deductible expense is the useful life of the item.

If the purchase is going to be used and no longer have value at the end of the reporting period it is an expense for that period. If the item is a capital item it is going to have a longer useful life. In this case the item is depreciated over its useful life, assigning an expense amount to each accounting period that the item has value.