The Apple Code Signing Identity “lets users trust that your app has been created by a source known to Apple and that it hasn’t been tampered with” (from Apple’s official documentation):
The following guide will show you how to create Code Signing Identity Certificate and use it in PubCoder to export an iOS App. To obtain a Code Signing Certificate, you need to have an Apple Developer account and be a member of the iOS Developer Program.
1- Log in Apple Developer Center, then click “Certificates, Identifiers & Profiles”.
2- Click “Certificates” under iOS Apps
3- Click on the “+” sign near the top right corner to add a new Certificate.
4- After that Select “App Store and Ad Hoc” Certificate
5- Create A CSR File, follow the page instructions. At the end of the process you will have a certificate file on your Finder.
6- Attach your .certSigningRequest file to this page via Choose File button and then press Generate button.
7- Download the generated certificate on your computer. Double click on it, this will be automatically added to your Certificates in your Keychain Access.
8. In PubCoder, under Project Settings open the Code Signing Identity (Distribution) selection, you should see now your new code sign identity among the options, this should have “iPhone Distribution” followed by your name and the Identity Code between parenthesis.