A disabling injury is denoted by severity; in that it directly impacts the ability of an employee to perform their normal work tasks, whereas a lost time injury simply means work time was lost due to an injury.
Minor injuries often cause lost work time. These injuries might include foreign object(s) in an eye (even common dust), minor lacerations, sprained fingers or ankles, etc.. Injuries may be quite minor or, on the opposite end, might be severe, but if work time was lost for the individual because of a work related injury, by definition it is a lost time injury.
In contrast, a disabling injury is denoted by severity and duration of time away from work. Due to the injury the employee is unable to perform necessary work tasks that they were able to perform prior to the injury. The disabling injury may be temporary, such as a broken arm, or permanent such as the loss of a hand.