If you develop a desktop application, then in order to sell it to another client, you will need to purchase licenses for the 1C:Enterprise platform for activation from the client, since the 1C application will not work without the 1C platform.
If you create a mobile application, then it does not require the installation of a platform, but for the commercial distribution of 1C mobile applications, you need to conclude an agreement with 1C company and pay royalties of 10% of the application's turnover.
If you distribute a mobile application for free (for example, as an additional feature to an accounting system), then you do not need to pay anything to 1C.
