You can extend existing elements with some additional parts to avoid having to edit existing files or overriding existing elements completely.

This is mainly used to append proficiency rules for new armor and weapons to the existing proficiency elements such as Ranged Martial Weapon Proficiency or Light Armor Proficiency. Also, you can append the name of a new spellcaster to existing spells this way.

Append

You can create an append node in a content file, anywhere where you normally would have an element. The append node takes an id attribute of the element you would like to add on to.

You can currently append supports and rules to existing elements. Some examples can be found below.

<append id="ID_OF_THE_ELEMENT_YOU_WANT_TO_EXTEND">
  <supports>Custom Support</supports>
  <rules>
    <grant type="Proficiency" id="ID_PROFICIENCY_SKILL_ACROBATICS" />
  </rules>
</append>
<append id="ID_PROFICIENCY_ARMOR_PROFICIENCY_MEDIUM_ARMOR">
  <rules>
    <grant type="Proficiency" id="ID_SCAG_PROFICIENCY_ARMOR_PROFICIENCY_SPIKED_ARMOR" />
  </rules>
</append>
<append id="ID_PHB_SPELL_BARKSKIN">
  <supports>Shaman</supports>
</append>
<!-- elves can be hill dwarfs -->
<append id="ID_SUB_RACE_HILL_DWARF">
  <supports>Elf</supports>
</append>
<!-- rogues always get free stealth proficiency -->
<append id="ID_WOTC_PHB_CLASS_ROGUE">
  <rules>
    <grant type="Proficiency" id="ID_PROFICIENCY_SKILL_STEALTH" />
  </rules>
</append>