Distance and Via

According to the help of the restriction editor (press the question mark in the dialog you show above)

The "Distance" slider controls how far to search for additional connected roads.

The "Via" slider adjusts how many via ways may be included in the search. (Tip: simple is better)

A turn restriction has a from and a to road. It also has either a via-node or multiple via-ways. So I assume that the above 2 controls will guide the iD editor in finding roads in case you want to create a turn-restriction with via-ways.

In case of a node, one can create the above turn restrictions, but when one moves the button slide to ways, one can create a turn restriction towards the parallel road when needed. This road is within a few meters of the selected one, so the top slider has no effect.

Hope this helps