What is a C&I circuit?
In this context it most likely means Commercial and Industrial. It is an umbrella term that covers lots of specific surge suppression types, but at least some are rated for Industrial use, the rest are commercial.
For industrial that translates to tougher enclosures, with 4x fiberglass and 4x stainless steel being top rated for harsh environments like bottling plants that handle citrus products, so the air is filled with citric acid, very corrosive to plain steel and aluminum.
Sewage treatment plants also need special enclosures and conduits for electrical power and control lines. C&amp;I is not a standard in itself as the NEC code book, UL and ISO standards define the details of assembly and assembly standards and documentation.
For some surge suppressor options and build and test standards go to: http://www.aptsurge.com/ Yes, I used to work for them for 15 years so maybe my choice is biased, but they are 'married' to UL and ISO standards as much as any surge suppresor manufacture. One could say they are a C&amp;I plant, in that they build both Commercial and Industrial surge suppressors.
In terms of everyday use of the words "Commercial and Industrial", commercial was used to describe products for residential use, including street lights. These often are pre-wired with wires a meter long or so and LED's for "Phase OK" indicators, up to 120Ka per phase. Priced under $500 USD if possible.
By default anything else was industrial rated, much more bulky with disconnect options, weight as much as 90 pounds, buss-bar connections and lugs for 8awg to 4awg wire. The term "C&amp;I" was seldom used, as we were implicit in conversations, meetings and documents about a products goal.
One could install an industrial rated surge suppressor in their home, but they cost thousands of dollars and would be the size of the breaker panel-or larger.