Dragging a Model under any Particles3D node types prompts to bind it to "parent" property.

Also, dragging ParticleSystem under any item that has the system property, such as ParticleEmitter, offers to bind it to system property. This is extremely rarely what user wants, as system to be bound is typically the parent of emitter or emitter is completely outside the system - it doesn't make sense to have emitters parent system as its child.