A reference collection is an extremely powerful and useful field type that can be added to any form. A reference collection is a set of fields from one form type added to another form type.
Let's say you have two different form types that you track - Daily Field Reports and Safety Inspections. Each form type lives on its own and is independent of each other.
You can go to one place to view the Daily Field Reports log and another to view the Safety Inspections log. But, since you'd like to see all Safety Inspections that occurred on any given day, you may choose to add a reference collection field to your Daily Field Report that references your Safety Inspections. That way, instead of going to different places to view information, you just view each day's Daily Field Report and get an overview of everything that happened.
When adding a new reference collection field, there are two additional properties you need to define besides field label and help text. They include:
Reference Form Type: Choose the other form type you'd like to reference.
Columns: Select the columns you'd like to show up in the collection.
Columns of a reference collection correspond to fields on the reference form type. The only field types that are not available to use as columns include other collections and multiline rich text.
Since each column is simply a field on the referenced form type, the access to the column is also derived from that field on the form type.