iOS Dual SIM some notes from Apple documentation

Phone XS, iPhone XS Max, and iPhone XR feature Dual SIM with a nano-SIM and an eSIM.1 An eSIM is a digital SIM that allows you to activate a cellular plan from your carrier without having to use a physical nano-SIM.

Some of the use cases are below


    Use one number for business and another number for personal calls.
    Add a local data plan when you travel outside of the country or region.
    Have separate voice and data plans.



Both of your phone numbers can make and receive voice calls and send and receive SMS and MMS.2 Your iPhone can use one cellular data network at a time.

Both of your phone numbers can make and receive voice calls and send and receive SMS and MMS.2 Your iPhone can use one cellular data network at a time.

1. eSIM on iPhone is not offered in mainland China. In Hong Kong and Macau, only iPhone XS features eSIM.
2. This uses Dual SIM Dual Standby technology, which means that both SIMs can make and receive calls. If one number is on a call, incoming calls on the other number will go to voicemail.

To use two different carriers, your iPhone must be unlocked. Otherwise, both plans must be from the same carrier. If a CDMA carrier provides your first SIM, your second SIM won't support CDMA. Contact your carrier for more information.

If you have an enterprise or corporate cellular service plan, check with your company administrator to see if they support this feature.


Below are the 3 options

Use Primary as your default line: If you select this option, Primary will be used by default for voice, SMS, Data, iMessage, and FaceTime. Secondary will be available just for voice and SMS.

Use Secondary as your default line: If you select this option, Secondary will be used for voice, SMS, Data, iMessage, and FaceTime. Primary will be available just for voice and SMS.

Use Secondary for cellular data only: You might want to select this option if you're traveling internationally and you want to keep Primary for voice, SMS, iMessage, and FaceTime. This will allow you to use Secondary for data.

References:
https://support.apple.com/en-us/HT209044