Holders are things containing, carrying or showing other things. What you place in them is revealed when the holder is action-tapped while standing next to it, or tapped on in your collection.
Holders have three cells: the 1st is the default, the 2nd shows the checked or opened state, and the 3rd is the tiling background when checked.
A single distinct Holder creation shows the same content across the world, whereever it is placed and may be viewed. Only the creator of the Holder can change its contents, but others can drag the content out of it to place or collect. The content can also be navigated with arrows and action-selected, resulting in the item's default behavior being triggered like in area possession bars.
You may want some things placed in your Holder to be fixed parts of it: to be unlabeled, non-selectable and non-movable when viewed by others. To do so either tick the "Non-Selectables" attribute for the Holder at large, or individually create things with the attribute "Holder Part" checked (this is available for Thing, Changing Thing, Big Thing, Interacting and Dynamic).
On Desktop, you can place all current Holder items in one go by holding Shift while dragging. The Holder needs to have the attribute Snap to Grid set for this.
A Dynamic placed in a holder has no display time limit of 15 seconds, and can be much longer.
An Interacting placed in a Holder reacts to the "Touches: ..." event when tapped or selected. It also fires a "Someone Holder-Selects ...: ..." event when that happens. To show a Holder, use "/shows holdername".