I’m not sure, but it looks like you might be confusing placeholder tags with content field/TV tags.
[[+placeholderName]] is a placeholder tag
[[*introtext]] is a content field/TV tag.
It’s easy to get these confused because many add-on components use placeholders to display content fields in their Tpl chunks.
See if this helps:
If you want to display a TV in a template, in a page’s content field, or in a chunk that is *not* being processed by an add-on component (e.g., getResources), you do it with a content field/TV tag:
MODx will look for a content field (e.g. introtext, pagetitle, content, etc.) with that name and for a TV with that name for the current resource.
If, instead, you are putting a tag in a Tpl chunk that will be used by an add-on component like getResources or Wayfinder to format its output, you usually need to make it a placeholder tag:
The add-on component sets the placeholders (often by getting the content field or TV directly from the DB). You need to consult the documentation for the addon to see which placeholders are set and what their names are.
Using a [[*field-name]] tag in a Tpl chunk, will still give you output, but it will display the value from the page containing the add-on tag (i.e., the current page) rather than the page the add-on is currently processing. That usually not what you want.