When reading the new provincial infection control guidelines, I see terms referring to different classes of treatment devices and instruments. What exactly do they mean?

A number of years ago, the Centers for Disease Control and Prevention (CDC) modified and updated the original Spaulding classification scheme for patient-care items. These categories termed, critical, semi-critical, and non-critical, were first developed by Spaulding in 1968 when he considered strategies for disinfecting and sterilizing medical and surgical instruments.  Dental instruments, devices, and equipment were included in later revisions. This Spaulding classification  continues to be used in infection control recommendations and guidelines. The categories are:
a. Critical items such as surgical instruments, periodontal scalers, and scalpels used to penetrate soft tissue or bone have the greatest risk of transmitting infection and should be cleaned and sterilized by heat.
b. Semicriticial items touch mucous membranes or nonintact skin and have a lower risk of transmission, but because semicritical items in dentistry are heat-tolerant, they should also by  sterilized by using heat. Dental examples here include mouth mirrors, handpieces, amalgam condensers, and reusable impression trays. If a semicritical item is heat-sensitive, it should at a minimum be processed with a high-level disinfectant.  
c. Noncritical patient-care items pose the least risk of infection transmission, contacting only intact skin. In most cases, cleaning, or if the item is visibly soiled, cleaning followed by        disinfection is adequate. 

Note: The recently approved dental infection control recommendations for Alberta call for cleaning and disinfection with an intermediate-level disinfectant, that is a hospital-level disinfectant (low-level) with an additional tuberculocidal claim. The guidelines published by the Royal College of Dental Surgeons of  Ontario recommend cleaning and subsequent use of a low-level disinfectant only.