Your ticket buyer has selected seats on the interactive floor plan. Now you need to pass those seats to your server-side code, so you can later on book them via the Seats API.

There are 4 ways to do this.

via selectedObjectsInputName.
If you add selectedObjectsInputName:"aFormFieldNameYouChooseYourself" to the seating chart configuration, the Seats Renderer will add a hidden input field with that name to your form. Note that the div in which you render the chart needs to be embedded in a <form> tag for this to work.
via chart.selectedObjects.
new seatsio.SeatingChart({}).render(); returns a seatsio.SeatingChart object, that has a selectedObjects property. This is a JavaScript array of strings that contains the selected object labels. Of course, we update the value on each select and deselect.
via chart.listSelectedObjects().
new seatsio.SeatingChart({}).render(); returns a seatsio.SeatingChart object on which you can call listSelectedObjects(callback) . The callback you pass in will be called on the list of selected objects.
by listening to onObjectSelected and onObjectDeselected events, and keeping track of the selected objects yourself.

Multi-level pricing

If you're using multi-level pricing, this means you have multiple ticket types (and associated price points) for a single category. For example, in Category A, a child seat is 5$ and an an adult seat is 10$.

This means you'll probably want to retrieve not just the selected seat ID (e.g. SectionX-A-1), but also the selected ticket type (e.g. "adult").

In that case, you can't use selectedObjectsInputName nor chart.selectedObjects, as they contain just the seat IDs.

Instead, you should use chart.listSelectedObjects(callback) . The callback takes the list of selected objects as a parameter, and the objects in the list have a selectedTicketType property, indicating the ticket type:

<div id="chart"></div>
<script type="text/javascript" src="https://cdn.seatsio.net/chart.js"></script>
<script>
    var chart = new seatsio.SeatingChart({
        divId: "chart",
        publicKey: "publicDemoKey",
        event: "smallTheatreEvent2",
        pricing: [
            {'category': 1, 'ticketTypes': [
                {'ticketType': 'adult', 'price': 30},
                {'ticketType': 'child', 'price': 20}
            ]},
            {'category': 2, 'ticketTypes': [
                {'ticketType': 'adult', 'price': 40},
                {'ticketType': 'child', 'price': 30},
                {'ticketType': '65+', 'price': 25}
            ]},
            {'category': 3, 'price': 50}
        ],
        priceFormatter: function(price) {
            return '$' + price;
        }
    }).render();
    
    function printSelectedObjects() {
        chart.listSelectedObjects(
            selectedObjects => console.table(selectedObjects)
        );
    }
</script>

<button onClick="printSelectedObjects();">
  Print Selected Seats to console
</button>