By default the user is able to:

Buy crypto using all available payment methods
Sell crypto to all available payout methods

By limiting the transferInMedia and transferOutMedia available, you are effectively limiting the available payment methods.
This is used to limit what type of trades (Buy/Sell) the end-user is able to perform.

See how to use transferInMedia & transferOutMedia to enable/disable Buy/Sell options below.

How to limit payment methods

Use case	Parameters	Comment
Disable Sell, allow buy via card or bank	`transferInMedia=card,bank`	Since `blockchain` is not available as `transferInMedia`, it's not possible to Sell crypto.
Disable Sell, allow all buy options	`transferOutMedia=blockchain`	The difference from the above example is that if a new `transferInMedia` become available, they will be enabled by default.
Disable Sell, allow buy bank only	`transferInMedia=bank`	Only bank transfer is allowed as the incoming transaction to pay for the trade. Similarly, if you use only `card`, the credit card payment becomes the only available payment option.
Disable buy, allow all sell options	`transferInMedia=blockchain`	If new transfer out media become available, they will be enabled.