This parameter is called order_id. It allows you to assign an identification number to your order (or to the product) that your client purchased, and then get a wallet address for paying for products by the customer, as you usually do in GCP.
 

This option allows to: 

• quickly identify your order with the wallet address that GCP generates and to which the payment is eventually received;
• easily maintain bookkeeping and internal reporting, where every item id corresponds to its wallet address.

Compare, if before you had to use the merchant_id (see "Flexible statistics in your account") when you wanted to identify items, now you can do it easier and simpler with the order_id, without changing the merchant_id or with using it for more general categories of items. 
 

Therefore, there are 2 parameters in GCP: 

• merchant_id which is a high-level identifier;
• order_id which is a low-level identifier.
   

So, order_id can be used for any item to be paid for, and merchant_id can be used for a category of items within which each has its own order_id. You can make it even simpler: the initial digits of the order_id will already have a category attached to it, e.g. "sporting goods" (in that case the merchant_id is not needed). Do it the way you like.

Also, previously, if you wanted to distinguish between products, you had to request the merchant_id and only then (specifying the merchant_id) —  request the payment address. Now order_id is specified immediately in the request for payment address. Setup has become easier and more user-friendly!

Cases of using order_id

You may use:

1. order_id only — identifying any order for which a payment is received;

2. order_id + merchant_id — where merchant_id is the name of the product group and order_id is any product in the group;

3. Advanced order_id — which already contains the category to which it belongs as well as other features of the product. For example, the product category is contained in the first 4 characters of the order_id, and the next 4 characters contain other features of the item (colour, size, country of origin, etc.).

Let's take an example of a complex business with:

• many groups and subgroups of products;
• many customers and, consequently, payments;

and preferably using order_id pro (improved) + merchant_id.

Case 1

We have several online casinos operating under different brands. Within each of them: different games, modes, users and, consequently, payments. To identify the payments, we can do the following: 

Level 1. merchant_id –> business name — Casino X

Level 2. order_id –> game mode — Double game (+ the number of the player, for example, depositing in this game)

Then it would be an advanced order_id, including both game mode and player.

Case 2

Let's go down a level lower (e.g. if we are inside the same site and there is no need to distinguish transactions from other businesses). 

Let's denote the game mode as merchant_id and the player number as order_id. This will be a simple order_id identifying the player only:

Level 1. merchant_id –> game mode — Baccarat

Level 2. order_id –> player number (+ his additional characteristics, e.g. what he bet on)