Watchers are presets that do not listen for Events firing in the browser but instead "watch" for geometry, position and surface of the specified nodes within the DOM node tree of the HTML document in the context of the rendered and determined being within the viewport targeted parts of the content.
Further, Watchers use the elements, styles and node structure patterns, scrolling and other conditions of the specified in parameters nodes as opportunities for insertion of ads (or secondary content) into the viewport of the viewer's browser and manage their positioning relative to the viewable content, including responsive layouts and programmed processes such as infinite-load and other interactive features.
LIQWID_board plays the role of a container for the LIQWID_Box [n] DIV — ad Box holder. This allows for flexibility to position the Box within the Board (for example Sticky Base placement can be set to create a semi-transparent overlay across the screen (Board) and position ad (Box) at the center or any specific side within the Board or aligned/centered relative to a specific node within the content. For the placements created for the commonly-used Rails/Aside columns on the right or left side of the content page, you may need sometimes to add one more container LIQWID_Rail [n] (that becomes parent node of the Board node) — this is useful in some complex layouts for a better managed positioning of the Board and its cloning/scrolling control.
It is important to remember the Targeted Node → [ Rail→ ] Board → Box structure as some parameters are specific to Targeted Node, some to Board, and some to Box for a wider pallet of possible placement positioning scenarios.
The main Target Node can be any string that can be understood by the querySelector method (w3schools.com.)
In rare occasions, you may need more than one Target Node selector. You can use more than one selector separated by pipe (|) or coma (for example: div.selector1, aside.selector2) because different nodes can be used on different pages and the placement Board will use the one it can find on the page (if it cannot not find any of the specified in the Target Node Selector nodes then it does not create any instance.)