After proto 1.3.3 and above, the order that you apply filters to regions affects their behavior. Here’s an example.

Let’s start out with a basic filter that denies everything except TNT.

<filter name="only-tnt">
    <block>tnt</block>
</filter>

Now let’s apply this filter to Region A, and also apply a filter to Region B that denies all blocks.

<apply block="only-tnt" region="region-a"/>

<apply block="never" region="region-b"/>

However, there is one slight complication. Region B is inside of Region A. So which filter will override the other? This is where the apply order comes in handy.

The order that you put the <apply>’s can change the outcome when something gets filtered. In the previous example, the only-tnt filter would always override the never filter since it is above it. The apply for region B won’t get checked since it overlaps completely with region A and region A never returns abstain.

If we changed the order of the filters:

<apply block="never" region="region-b"/>

<apply block="only-tnt" region="region-a"/>

Now you cannot place blocks in Region B, but outside of it in Region A you can only place TNT blocks.

Although this may not be an issue for many maps such as a Blitz map or a TDM, this is a crucial concept for many DTC/DTM/CTW maps that use the <void/> filter. Usually it’s best to have the <void/> filter at the lowest priority (at the bottom of the list).

A map developer will be able to help you through this process to ensure that your map’s regions and filters are properly working.