User defined properties are very helpful to store some simple data (a string or number). You can define your own properties in the Custom Property View, or define it dynamically via the Set Global Property action.
To insert an element property into expression, you can click the button beside the input field and choose "Insert User Defined Property..." in popup menu.
Then you can choose the user defined property from a drop-down list. You will see all previously defined properties here:
If you need to store data structure with 1 dimension (array) or 2 dimensions (table), you may consider storing them with an (invisible) Table element. You can use Set Table Cell Value, Append New Row to Table, Insert New Row to Table and Delete Row from Table actions to manage the data, and use the Table Row Count and Table Cell Values properties to retrieve your data.