There are several ways that an STP brokerage can make a profit. The broker may either charge their clients a commission ($X per $million), apply a markup to the price feed, or perhaps a combination of both.
In MT4, the commission charged to the client is typically set in deposit currency per lot ( In this article, we will be using US Dollars as deposit currency for an easy and clear explanation).
An STP brokerage can also choose to charge their clients a markup, which will be added into spread that their retail clients see. Markup is normally defined as points added per lot. Markup is usually invisible to your clients and tends to be more flexible as you have the ability to adjust the markup according to market conditions.