Using the underscore when creating control fields

Can somebody explain to me the meaning of using an underscore and the plural form (‘tags._ids’) in the line " echo $this->Form->control(‘tags._ids’, [‘options’ => $tags]);" ,

Search through this page for references to _ids.

Thank you for the reply.
Maybe a hint at the linked page directly in the line where the control field is created in the tutorial could be useful for newbies.

The documentation is under source control, so you could certainly make a pull request adding this.