A reference selector does exactly what the name says it does – you select a reference. Say you have a page on which you have a data view of a specific employee and you put a reference selector on the page to select which company they work for. Once you select it, the association between company and employee is set.
Now, if you want to use that same company in a microflow, simply do a retrieve over the employee_company association, and you'll be able to use the selected company in your microflow.
You can use an on change event for this and call a microflow that will save the value on a persistent entity. You can retrieve the value over association from your context object. The context (a non persistible object) from the page will be automatically passed to the microflow as a paremeter.
Or you have a custom save action on the entire context object. It all depends a bit what you exactly want to achieve.