To understand how commissions are charged on the exchange, you need to distinguish between maker and taker fees:
Maker fees are applied to market makers providing liquidity to the market.
In a nutshell, a market maker is a trader who places an order that is listed in the order book (partially or in the full amount). A prime example of this is a limit order which awaits execution until the market hits the specified order price and may be executed in portions.
In effect, such orders add up to the volume available in the order book (thus assisting in “market making”). For this reason, all trades executed as part of filling such an order are considered market maker trades.
Taker fees are applied to market takers diminishing market liquidity.
A market taker is a trader who places an order that is executed immediately, without being listed in the order book. Regardless of whether such orders are executed partially or in full, the corresponding trades diminish the order book volume.
Since market orders are never listed in the order book, any trades that are executed upon placing such orders (IOC and FOK market orders) are considered market taker trades.
From the standpoint of the exchange, it is reasonable to encourage market makers by charging them lesser fees with a prospect of increasing liquidity on the exchange.
Commissions that are due to be charged for trades executed by users can be defined at different levels:
Custom maker and taker fees can be specified for different user groups. These fees have the highest priority and are charged regardless of volumes traded by users included in these groups.
Tier commissions are charged depending on pre-defined volumes traded by users across selected markets within a 30-day period. These settings have a higher priority then the fees specified for each market (see below).
Market fees are defined individually for each market. By default, these fees are only charged for all trades on a market if no tier and custom commissions are specified for it.
A user can be added only to a single commission group. After adding users to a new commission group, they are removed from the one to which they were previously assigned.
Both the exchange users and commission groups are created and managed by the platform administrator.
If a user is added to a custom commission group, the Role column indicates its name.
The maker and taker fees defined for custom commission groups are charged regardless of the traded volume (as opposed to the tier commissions that are only charged when trades meet specific volume requirements).
The fee amount is calculated for the asset that is purchased by a user upon a trade execution (as opposed to the asset that is being sold).