Meaning per category - J
This category consists entirely of warning signs. These signs give you the opportunity to timely anticipate possible danger. Sometimes they are combined with signs from category A, this can be with both a speed limit as well as with a recommended speed.
For example, there are signs that warn about a certain condition or shape of the road:

J-1 - Poor road surface

J-2 - Bend to the right

J-3 - Bend to the left

J-4 - S-bend, first to the right

J-5 - S-bend, first to the left

J-6 - Steep slope

J-7 - Dangerous descent

J-16 - Roadworks in progress

J-17 - Carriageway narrowing

J-18 - Carriageway narrowing on the right

J-19 - Carriageway narrowing on the left

J-20 - Skidding danger

J-25 - Loose chippings

J-26 - Quay or riverbank

J-38 - Speed bump

J-39 - Retractable bollard (poller) in the carriageway
In addition to that, there are signs that warn you that you are approaching a dangerous traffic point:

J-8 - Dangerous intersection
(usually placed at dangerous equivalent intersections)

J-9 - Roundabout

J-10 - Level crossing with crossing barriers

J-11 - Level crossing without crossing barriers

J-14 - Tram (crossing)

J-15 - Movable bridge

J-32 - Traffic lights
St. Andrews crosses are always placed at level crossings (indicated by signs J-10 and J-11). These indicate how many tracks cross the road. With a single St. Andrews cross (J-12) there is just one track, with a double St. Andrews cross (J-13) this can be two, or even more tracks. In that case, another train can approach from the other direction after one has already passed.

J-12 - Single St. Andrews cross (one track)

J-13 - Double St. Andrews cross (two or more tracks)
The following signs warns you of other road users, animals and low-flying aircraft:

J-21 - Children

J-22 - Pedestrian crossing

J-23 - Pedestrians

J-24 - Cyclists and moped riders

J-27 - Big game (wild animals)

J-28 - Livestock

J-29 - Oncoming traffic

J-30 - Low-flying planes
Do note! Even though the J-24 sign only displays a bicycle symbol, it also warns of moped riders.
The following signs are typically used temporarily on electronic matrix signs. This means that they only appear when the situation arises. But they can also be a standard where the situation regularly occurs. For example, where traffic jams often occur, or on viaducts where there is almost always a crosswind.

J-31 - Crosswind

J-33 - Possible traffic jam

J-34 - Accident

J-35 - Poor visibility due to snow, rain or fog

J-36 - Sleet or snow
If all these signs cannot represent the danger, then there is a general danger sign. The danger itself is shown on the supplementary plate:

J-37 - Danger, rutting

J-37 - Danger, exit construction