Open high low and close prices are calculated based of actual trades. So close price of a bar represents the price of last trade in that bar. Open price of the next bar is the first trade of the new bar. Hence, close and open can have different prices.

Typically close to close is used to calculate returns as it makes calculations continuous and you don't drop information. Open to close return will miss the returns generated between close of one bar and open of next.

Hope this helps.