According to Czech law, every company must register its sales electronically (CZ: EET - Elektronická evidence tržeb). In this tab you will find basic info about it. Here you can fill in several windows, you set the business identifier on the Tax Portal, where you also download the certificate and create a password for it. When you click Select File, the system will prompt you to select the correct .p12 file in front of the File Explorer window, and when you select it, it will be loaded. After entering the password and store ID, you can click Submit.
Everything about EET can be found in a separate section here.