Short calls are calls that disconnect prior to the "Abandoned Call Wait Time". You can change that setting here:
Configuration Manager:
Configure ICM -> Enterprise -> System Information -> System Information
Abandoned call wait time
The way I understand it, "Incomplete" are calls that didn't connect to a resource like a trunk or an agent. So maybe the script attempts to send the call to an agent but the label is incorrect or the call drops in process, that would be incomplete. Or an agent dials an invalid number for an outgoing call.