An order can be in the status of Completed, Corrected or some other status. I saw some code that is checking it like this, the purpose is to disable some stuff when the status is in Completed or Corrected status.  if (model.CurrentStatus != DSRHelper.OrderStatusEnum.Complete && model.CurrentStatus != DSRHelper.OrderStatusEnum.Corrected)I can't get it why the engineer has used AND for this, shouldn't it be an OR? 
Should I use AND or should I use OR
conditions;programming logic
No, AND appears to be correct here. If you switched it to OR, the condition would always be true. If it was Completed, then the second condition would be true. If it was Corrected, then the first would be true. Keep in mind that the condition currently used is equivalent to!(model.CurrentStatus == DSRHelper.OrderStatusEnum.Complete || model.CurrentStatus == DSRHelper.OrderStatusEnum.Corrected)