Using templates and slots - Web APIs | MDN - MDN Web Docs

mdn slot When the slot's containing component gets rendered, the slot is rendered with the custom element's child that has a matching slot attribute. A named slot is a slot> element. The slot property of the Element interface returns the name of the shadow DOM slot the element is inserted

mdn slot, The slot property of the Element interface returns the name of the shadow DOM slot the element is inserted.