A null object is a generic, empty object. They’re used in different programs for different purposes, but they’re often used as parent objects to control a bunch of children in a unified way. To scale them uniformly, for example, or to move them all to the right in unison while they each make other, independent, movements.
In motion you could simply use an empty group or layer. Or you can place all of the children into a single group or layer & apply your transforms to that group or layer.
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