The XO Type test tool is used primarily to pressure test tubing. 

The XO test tool can be placed into selective and run to the bottom most profile nipple of corresponding size and latched into the locking profile to provide a secure means of pressure testing the tubing string.

The XO Test tool would typically be run in selective to the bottom most XN no-go nipple and be inserted into the seal bore, once testing is complete it can be moved up hole to test the tubing string should a leak be suspected.

Applications:

The XO type test tool can be used to pressure test the integrity of a tubing string.

The XO type test tool can be used to selectively pressure test the tubing string should there be a leak suspected, this is accomplished by moving the test tool uphole and setting in corresponding X profile until a leak is isolated or ruled out.