If you have exported a report from analytics, you may find a few IDs and a set of characters that may confuse you. Let's take a look at what these parameters are:
ui - User ID: A randomly generated set of characters representing the user who viewed the event. This ID is not unique across all users because it doesn't contain any private information about the user. But it's unique enough to be used when displaying the "number of unique visitors that viewed the story".
pid - Product ID: A set of characters representing a product.
cid - Customer ID: Similar to the product ID, but for the customer. This ID is used for audience targeting when displaying personalized stories.
sid - Serial ID: Refers to the serial number of the product.