When adding a partition, should I expect to lose data or not? I'm trying to understand what is meant by the wording of this Disk Utility cautionary message that appears when attempting to add a new partition to a disk.

Partitioning this device will delete the data stored on some of the partitions, 
and cannot be undone.

This partition will be added:

"Secondary"

This partition will be erased:

"Secondary"

This partition will be resized:

"Primary"


First point of confusion: It's going to be added and then erased?

Second point of confusion: Disk Utility is supposed to be able to add, resize, and delete partitions without losing data. Sources: 1, 2 So why the ominous message, and who to believe?

Update

I added a new partition and everything appears to have gone smoothly.
An answer to another similar question helped me to understand and decide to move forward with partitioning.

This guide, How to Use macOS Sierra Disk Utility to Partition, Erase Drives was also helpful.
Data should not be lost.
Just as Solar Mike said, do a backup if you can.

It creates a second partition, but the second will be empty, without any file system, so it will be erased to format it into the file system you choose.