The data can often be divided into units of a specific type and certain characteristics.
In addition, some types of units are bound by certain relationships with others.
For example the invoice is characterized by number, date.
The invoice contains the items.
Each item describes a specific product.
A common feature of these data is that they can be represented in the form of
several tables with a certain number of columns of specified type.
The rows in one table can be related with rows in another.

Typically relational databases contain different descriptive and numeric data
eg. invoices, inventories, stories, accounts, customers, employees, parts, etc.

Relational data is often physically stored in files on disk.
However, the files are organized in a way that
determines the order of items and records the relationship between them.

But it should be remembered that it can be used to store any data,
for which relations havae sense. For an example you can in this way
save images, documents, movies and even "assemblies" from the executable code of the program.
Maps are an example of where it is possible to store graphics in a relational way.
It is important that a theoretical ideas can be used in many different ways.