You can define accessible seats from the floor plan Designer via two methods:
Selecting the seats and marking them as Accessible
Creating an accessible category and applying it to seats
Defining Accessible and Companion seats
You can specify seats as accessible or for companions of such attendees. To do so, from the Designer choose the Seat selection tool:
Then, proceed to select one or multiple seats, and finally on the right hand of your screen, where the object properties are listed, turn on Accessible or Companion seat as needed:
Defining and applying Accessible categories
From within the Designer, create a new category, or edit and existing one by right clicking it and choosing Edit, and turn on the Accessible setting for it:
Then, select rows using the Select tool, or individual seats using the Select seats tool, and apply the category using the category picker from the inspector.
