Insert (Circuit menu)
The Insert row in the Circuit menu opens a flyout listing every other circuit and sub-circuit in the project that can be placed as a live instance on the active canvas — the same flyout is also available from the canvas' right-click menu when nothing is selected. Picking one drops a package instance centered in the current view, wired to the same live circuit it represents.
What appears in the list
The flyout lists every top-level circuit and sub-circuit in the project except main and the circuit you are currently viewing, so you never accidentally nest a circuit inside itself. If the project has nothing else to offer, the row is disabled. Click a name to insert it.
Why an entry might be missing or blocked
A circuit needs at least one Input or Output pin before it can be inserted — one with no exposed pins is filtered out of the list, since there would be nothing to wire on the instance. A circuit can also never contain itself, directly or through a chain of insertions, so anything that would create that cycle is excluded too. And because a sub-circuit cannot itself insert other circuits, the whole flyout is disabled while you are drilled into one.