Trapping is described in Wikipedia (http://en.wikipedia.org/wiki/Trap_%28printing%29). In short: Trapping means to define information for the printing process to avoid uncolored paper after printing a multicolored document. The Wikipedia article provides further links to detailed information from Adobe.

A PDF document contains the information about whether it is “trapped” or not. Adobe Reader® shows the property “Trapped” in the document properties dialog:

“Trapped” can have the values Yes, No and Unknown. Only one tag is provided to verify this property:

<!-- Tag to verify trapping information: -->

<hasTrappingInfo value=".."  (required) 
/>

<!-- 
  Only these constants are allowed for the attribute 'value': 
-->

value="YES"
value="NO"
value="UNKNOWN"

<testcase name="hasTrappingInfo_Yes">
  <assertThat testDocument="trapping/trapping-info_yes.pdf">
    <hasTrappingInfo value="YES" />
  </assertThat>
</testcase>

<testcase name="hasTrappingInfo_No">
  <assertThat testDocument="trapping/trapping-info_no.pdf">
    <hasTrappingInfo value="NO" />
  </assertThat>
</testcase>

<testcase name="hasTrappingInfo_Unknown">
  <assertThat testDocument="trapping/trapping-info_unknown.pdf">
    <hasTrappingInfo value="UNKNOWN" />
  </assertThat>
</testcase>