What is best practice for mapping signal controlled pedestrian crossings at road junctions

I've never quite understood the way in which signal-controlled pedestrian crossings (in the UK pelicans, toucans etc.) are tagged. This is fine when I'm using JOSM as there are pre-sets, but when I'm using Potlatch I find the exact convention used in JOSM difficult to remember. So to start with I'm never sure whether to use highway=traffic_signals or highway=crossing together with crossing=traffic_signals.

At road junctions when the traffic signals control vehicular traffic as well as the pedestrian crossing I get more confused. For a simple junction (two streets crossing) I could leave a highway=traffic_signals on the junction node, and tag the pedestrian crossings as highway=crossing, or I can remove a single node for the traffic signals and rely on tagging each set of lights together with their pedestrian crossing attributes.

I would like:

• a description of the standard practice of tagging for signal-controlled crossings (if your answer is fully covered by existing responses to this question how-to-tag-traffic-lights-on-pedestrian-crossing, please avoid duplicating answers);

• an explanation of why a particular combination of tags are used;

• how this should be applied to junctions where the signals are controlling both vehicles and pedestrians;

• and, lastly, advice on linking together traffic signals at junctions (e.g., with a relation). Please don't forget that most cases of junctions are more complex than just two streets crossing.