Complex sentence: When the CSA mark is shown alone or with a "C" pre-fix, it identifies that the item is certified for sale in Canada; when also showing a "US" mark, the item is also certified for sale in the United States.
Split into simpler sentences: When the CSA mark is shown alone or with a "C" pre-fix it identified the items is certified for sale in Canada. When also showing a "US" mark the item is also certified for sale in the United States.