Overlapping zones
Zones can overlap. Overlapping zones on different layers don’t affect each other. If they belong to the same net, you can stitch them with vias; otherwise not.
Zone borders can also overlap on the same layer.
The behavior in that situation depends on the properties of each zone. Naturally the net is important. Another one is priority.
Zones with different nets
When zones belong to different nets, KiCad locigally can’t just know how they should be filled if you haven’t given information about which one should take precedence. If they have the same priority value, they are both just filled. This leads to a situation where you have a DRC violation.
Therefore you have to give them lower and higher priority values (lower value = lower priority). The higher priority zone is filled over the lower priority one, and the lower priority zone isn’t filled in the overlapping area.
Clearance values
Each zone has its own clearance value. This doesn’t override the net class clearance (which is set in the Board Settings). They are both used and the larger one is effective. Thefore, if you have two zones of different nets which have the same priority, the gap between them must be at least as wide as “max(zone1_clearance, zone2_clearance, net1_clearance, net2_clearance)” (i.e. the largest of the four clearance values).
Fine tuning the gap can be avoided by setting the priority values. Then the lower priority zone avoids the larger priority zone and the largest clearance value is effective.
Zones with the same net
If you draw two overlapping zones with the same net and the same priority, KiCad may fuse them together. (The behavior seems to be a bit unpredictable here in version 5.1.4, but you can’t expect them to remain as two distinct zones.)
Zones with different priorities are kept separate, but as of 5.1.4 the result when filling may be something you don’t want.
