As tastrax says, OSM is a database with tags. There are many contributors and people don't always use the same tagging methodology. This makes it difficult to have any consistency when producing maps from OSM.
Converting OSM tags to Garmin styles is equally complicated. The final display is determined not just by tags, but also zoom levels.
I inherited a set of styles but I'm not quite sure where from. I haven't made many changes. It is quite time consuming. Make a change, regenerate the map and have a look. The change may look better for one area, but have undesirable consequences somewhere else where the tags are different.
Basically the legend hierarchy is as follows:
Yellow solid, grey outline: secondary
Orange solid, orange outline: tertiary
White solid, grey outline: residential/service
Red dash: unclassified
Brown dash: track
Red dot: footway/path
There are tags that can be used to distinguish 4wd tracks, but IMO they are not used widely enough to be useful/reliable.
My priority was to produce maps for bushwalking. If I want to know about access road classification I would use the relevant states online mapping.
peregrinator wrote:In the OSM Wiki legend above, all vehicular bush tracks are shown as brown dashed lines. For andrewp's Vic, NSW and SA maps, such tracks are shown as either brown or red dashed lines. What's intended by that distinction? I'm guessing that brown is 2WD and red is 4WD, but I'd like to be sure.
To answer your specific question. Red dash roads are tagged as highway=unclassified and brown dash roads are tagged as highway=track. I.e red dash is more likely to be 2wd and brown dash is more likely to be 4wd. Where were you looking at when you came to the opposite conclusion?