Ukelele keylayout file: What is keyMapSet id and what is it used for

Question

I have a .keylayout-file produced by Ukelele which contains keyMapSets.

Can anybody explain to me how this works?

• Why / for what purpose do we need to define different mapSet (16c and 984 here)?
• "first"/"last" are the keys that are affected. But where are the missing keys 19+20, 24-29, 31++ ?
• Why are there different definitions for the keys

•    e.g. **key code="24" output="="** (for 16c) vs **key code="24" output="^"** (for 984)
  

• When would I use which definition?

---

keyboard group="126" id="-15908" name="U.S. copy" maxout="2">
<layouts>
    <layout first="0" last="17" mapSet="16c" modifiers="f4"/>
    <layout first="18" last="18" mapSet="984" modifiers="f4"/>
    <layout first="21" last="23" mapSet="984" modifiers="f4"/>
    <layout first="30" last="30" mapSet="984" modifiers="f4"/>
    <layout first="194" last="194" mapSet="984" modifiers="f4"/>
    <layout first="197" last="197" mapSet="984" modifiers="f4"/>
    <layout first="200" last="201" mapSet="984" modifiers="f4"/>
    <layout first="206" last="207" mapSet="984" modifiers="f4"/>
</layouts>

modifierMap 
...
</modifierMap>

<keyMapSet id="16c">
    <keyMap index="0">
        <key code="0" action="a14"/>
        ...
        <key code="24" output="="/>
        ...
    </keyMap>
    <keyMap index="1">
        <key code="0" action="a6"/>
        ...
        <key code="24" output="+"/>
        ...
        <key code="126" output="&#x001E;"/>
    </keyMap>
</keyMapSet>
<keyMapSet id="984">
     <keyMap index="0" baseMapSet="16c" baseIndex="0">
        <key code="24" output="^"/>
        <key code="30" output="["/>
        <key code="33" output="@"/>
        <key code="39" output=":"/>
        <key code="42" output="]"/>
        <key code="93" output="¥"/>
        <key code="94" output="_"/>
        <key code="95" output=","/>
        <key code="102" action="a5"/>
        <key code="104" action="a5"/>
    </keyMap>
    <keyMap index="1" baseMapSet="16c" baseIndex="1">
        <key code="19" output="&#x0022;"/>
        <key code="22" output="&#x0026;"/>
        <key code="24" output="~"/>
        <key code="25" output=")"/>
        ...
    </keyMap>