In order keep configuration simple, please perform the following steps in Salesforce Classic (Booker25 is Lightning-Ready, you may afterwards use the app in Salesforce Lightning as you please)
After you have configured dimension junction objects you might want display this related list on the reservation form and allow users to add new records. To do this you have to create a Dimension Junction Display Setting object to each Reservation Type you want to display this related list on. The Dimension Junction Display Setting object has some additional fields that allow you to control certain aspects of this related list.
Field
Type
Required
Description
Dimension Junction
Lookup
Yes
The dimension junction this Dimension Junction Display Setting displays
Reservation Type
Lookup
Yes
The Reservation Type this Dimension Junction Display Setting displays this Dimension Junction on.
Field Set to use
Text
No
The fieldset on the Dimension Junction object to use to display extra field in the dropdown section below each related item. If left blank the dropdown section is hidden.
Order
Number
No
When multiple related lists are shown on the reservation form the related list with the lowest order wil be shown at the top. Blank values are treated as last.