Hi Oskaras,
Great question!
In the Utility Network data model, you have two preconfigured set of feature classes. You have your Domain Network and your Structure Network. Domain networks contain the features through which your resource flows (hot water, steam or chilled water pipes). Structure networks do not have resources flowing through them; however, a structure can contain other features that are part of the domain network. For example, you could have a concrete trench box or a tunnel (structure line) that houses your hot water (or steam, chilled water) pipes.
A utility network can have multiple domains (distribution, transmission, generation for example), but only needs one structure network.
Your regular line feature class that would house your pipes would be in your domain network and your structure line feature class would house the structures that hold the pipes such as a casing, tunnel or trench box.
For more detailed information on the differences between a Domain Network and a Structure Network you can look at the following reference guide:
https://pro.arcgis.com/en/pro-app/latest/help/data/utility-network/what-is-a-utility-network-.htm
Specifically, the section under Utility Network concepts which will give you a detailed look into the differences between the two sets of feature classes.
If you have additional questions please do not hesitate to reach out to us!
Jess Gooch, Solution Engineer, Utilities
