There are THREE types of Static Control Garments that are compliant to the ANSI/ESD S20.20-2014 Standards.
Steve Cooter of United Static Control Products explains the Levels on their website. Below is an excerpt:
1) Static Control Garment – Level 1
These smocks are primarily designed to shield the technician’s clothing from the ESD Sensitive product. They DO NOT come with a ground snap and DO NOT require a ground cord. When these static control garments come into contact with the users skin (the user’s skin is about as electrically conductive as the garment) the static charge is equalized and goes to ground via the technician’s primary ground source (normally an ESD wrist strap and or ESD heel grounders).
2) Groundable Static Control Garment – Level 2
These smocks are primarily designed to shield the technician’s clothing from the ESD Sensitive product but they also provide redundant protection in the unusual event the smock does not make contact with the users skin. The ESD smock has a snap on the hip that can connect a ground cord to create a path to electrical ground. A Groundable Static Control Garment is NOT recommended as the technician’s primary ground source (an ESD wrist strap and or ESD heel straps are still required).
3) Groundable Static Control Garment System – Level 3
These smocks are designed to act as a primary ground source for the technician (they can act as a replacement for a wrist strap). In addition to grounding the technician, they will also help to shield the technician’s clothing from the ESD sensitive product. They are reliant on good skin to smock contact. Hence, they may come with ESD cuffs or snap style cuffs that fit snugly around the users wrist.
United Static Control Products has been writing about ESD control since 1995 and their website has over 400 pages of information. Their line of ESD jackets can be viewed here.