tek
stands for Tektronix indeed. Although you didn't provide the resulting Xliff, but I guess the reason that you're not able to validate it against the Xliff core schema is that your document has two namespaces; the original Xliff namespace and a custom namespace called tek
which is used to add customized vocabulary to the file.
Here is a foo
example:
<xliff xmlns="urn:oasis:names:tc:xliff:document:1.2"
xmlns:foo="http://www.foobar.com"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="urn:oasis:names:tc:xliff:document:1.2
xliff-core-1.2-strict.xsd http://www.foobar.com foo.xsd"
version="1.2">
Then you can have:
<trans-unit id="0" restype="button" resname="big_red_button">
<foo:serious-note>Srsly!</foo:serious-note>
<source xml:lang="en-US">Don't push here!</source>
<target state="needs-translation></target>
</trans-unit>
And as long as you add this customized tag to your schema, you can validate the file using both Xliff schema and your customized schema:
<xsd:element name="serious-note">
<xsd:complexType>
<xsd:simpleContent>
<xsd:extension base="xsd:string">
<xsd:attribute ref="xml:lang" use="optional"/>
<xsd:attribute name="foobar" type="xsd:string" use="optional"/>
</xsd:extension>
</xsd:simpleContent>
</xsd:complexType>
</xsd:element>