Usually u5CMS-commands – e. g. [b]bold text[/] – must be outside HTML-sections:

Here u5CMS-code like [b]bold text[/] is possible
[h:]
Here you are inside an HTML-section, therefore [b]bold text[/] does not
work but <strong>bold text</strong> (or old style: <b>bold text</b>).
[:h]
Here we are outside the HTML-section again.

The command for inclusions [$$$:nameofthepagetoinclude] – e. g. [$$$:datepicker] – and the command for formdata queries [fieldname|formname:dat] and references to objects – e. g. [imagename] – and further superglobals are useful exceptions, they do work outside and inside HTML-sections.

That's why you can simply type [$$$:datepicker] within an HTML-section, instead of stopping the HTML-section first, entering the u5CMS-command for an inclusion then, and restarting the HTML-section afterwards (what would look like this: [:h][$$$:datepicker][h:]).