How should these points be understood in the development plan?

My goodness, what have the urban planners come up with this time. This could be the final exam in the advanced German course: Interpret the text and work out the author's intentions.

basically, whoever digs out or fills up has to secure it, right?

Yes.

you are allowed to do whatever you want within the property

No. "The natural terrain is to be largely retained"

when is a terrace elevated and when is a slope steep?

You don't know. You could say anything higher than the surroundings is elevated and anything steeper than the natural slope ratio of 1:1.5 is steep. Maybe the justification for the development plan says something. The regulation itself is vague, unless something from the unpublished points helps.

(but only 20cm high)

Was that said? But it is not apparent from the regulation.

that base walls of fences are only allowed to be 20cm high and natural stone walls are permitted

I agree with that too.

how are you supposed to compensate for height differences then?

That only concerns the enclosures, which are usually located at property boundaries where height differences have to be overcome by slopes.

then neighbor 3 wouldn’t be allowed to have a fence at all because due to the embankment it’s already over 120cm

There must be no embankment on the boundary, otherwise he would be violating the development plan. If he has an exemption, it will probably also apply to a fence. Or those who approve are just as nonsensical as those who plan.

are no retaining walls allowed at all? at least on the boundaries?

Yes.

not even the 20cm???

20cm base walls are allowed, but not for retaining.

are planter rings considered a slope?

No.

no form of wall whatsoever

Yes. As an enclosure, but not as a retaining wall.

to have a flat area in front of the entrance

For what?

am I allowed to build a wall with some distance to the sidewalk (Max, EFOK) and have an area between the wall and sidewalk at sidewalk level?

Yes, unless the local and landscape image is impaired. Which brings us back to the German exam.

so 30cm height over one meter

That would be 30%. At 30°, the height difference is 0.58m.

thus, one has to "sacrifice" 2 meters of their property at the corresponding boundary if they want to correct 60cm.

Starting from the natural slope ratio, it would be 90cm for a 60cm height difference

That would be 30%. At 30° the height difference is 0.58m.

Based on the natural slope ratio, it would be 90cm for a 60cm height difference

sorry, I can't get the quotes right. but Escroda's post is slowly bringing some clarity for me

So dry stone walls may be up to 120cm high as fencing, base walls of fences 20cm, no walls etc. for support, here slopes must be made
within the property walls (probably only dry stone walls) are allowed as long as the natural terrain profile is largely maintained

if this were a German exam, I would have gotten a 6

So dry stone walls may be up to 120cm high as a fence, [...] if this were a German exam, I would have gotten a 6

At least this paragraph would be heavily crossed out in red. Only fences (or nothing) are allowed as enclosures; the bases of the fences may always be only 20 cm high, regardless of whether they are concreted, wet or dry masonry. Only fences may have a total height of 120 cm; within the property, EFOK is king. Within the property, you can make grass slopes or natural stone slopes; the use of planting rings I do not read as excluded. The terrain profile should remain similar to the original, so at most shaped and not violated.

but under 3. it says about enclosure: Dry stone walls made of natural stone are permitted

what am I allowed to do with it?

In case of doubt, stack the natural stone only up to skirting height, without using it as the base of a fence (?) - what does say: has definiteness been neglected here?