Hackolade, being built on JSON Schema, leverages this little-known but extremely powerful feature of JSON.
Pattern fields function in exactly the same way as standard fields, with one exception: the name (in the name-value pair) is not a fixed string, but a regex pattern.
This is represented in the Hackolade ERD as:
The name in the name-value pair is not a fixed word, but a variable. That's what pattern fields are for. They are typically controlled by a regular expression, hence the name of pattern field.
You should not look at it as a 'field name', but as a key to a sub-document when embedding. It lets you easily access the right sub-document inside an array or main document, using dot notation.
In Hackolade, the above example is modeled this way:
To add a pattern field to a document structure, right-click and choose add/insert/append > Pattern Field then the field data type: