Data Modeling is an essential and important step for any project. Data modelling is always done in items.xml.

items.xml placed under resource folder in every extension with naming convention given below:

Syntax : < extension-name > – items.xml
Example : core-items.xml

Note: core is an extension in commerce cloud.

Lets understand like this:

For Example , we are developing a website which is operating Customers and Products. These customers must have some feature: like Name , DateOfBirth , Address and and so on.
Products will have also some features like: Price, Color, name , stocks and so on.

So in this case Customers and Products will have tables in Data base and these table must store the attributes as given above.

In Terms of JAVA  :

Class : Product
Object : mobile
Variables : price, color, name , stocks

Class : Customer
Object : Mr.XYZ
Variables: name , dateOfBirth , address

In Terms of SAP Commerce Cloud:

type : Product
item : mobile
attribute: price, color, name , stocks

type : Customer
item : Mr.XYZ
attribute : name , dateOfBirth , address