@rametto
To find the type and subtype of an object , I use special typ files. These typ files contain objects (lines for example) with a special color : the type/subtype is written in the hex code of the color.
So you just have to look at you map in BaseCamp or Mapsource and if the line has changed, it means that the code is in the typ file. Read the color of the line and you have the typ/subtype.

Step by step
- use TYPViewer to create such special typ files : "Tools" --> "Special TYP for types identification" select the type of the object and the range of types. For example, most of the lines are in the ranges 0x00 to 0xFF with no subtype or 0x10000 to 0x10f1f or 0x11000 to 0x11f1f . Then click on "OK"
- use TYPViewer to deactivate shading in Basecamp/Mapsource (otherwise colors will be modified by the shading) : "Tools" --> "Activate/deactivate shading in BC or MS
- associate your special typ file to your mapset : "Tools" --> "Install a TYP file..." : select your mapset in the list then click on "Choose a TYP file" select your typ file and then "OK"
- open Basecamp or Mapsource, go to a location where you can see your line, in TYPViewer go to "Tools" --> "Color identifier" and put the cursor over the line : the little window of the color identifier will show you the type/subtype of the line


@giomen
you cannot remove a polygon with a typ file but, as you say, you can make it transparent. (playing with priority is not a good thing for hiding polygons...)
Another way : if a mapset is associated to a typ file and if this typ file has a draworder section that is not empty, only the polygons written in the draworder are displayed. So , if you remove a polygon from the draworder, it will not be displayed.
The second method is perhaps a better way to do this, because the polygon is not displayed at all (with the first method, it is displayed but as a transparent one) so the GPS unit will be less slowed down IMHO...