I believe the suggestions below that
tan here means
tangent are correct. The reason for that is checking out how it is used in technical engineering forums.
Check this one:
http://www.eng-tips.com/viewthread.cfm?qid=194779&page=88Which has a question:
"If an insert plate is used for a nozzle which is very close to the
tangent line can the insert plate edge have a rectangular shape at the edge at which it will be welded to the dished head?"
For which it gets the answer:
"The dished end should be selected with a length of skirt, to avoid welding on/near the
tan line"
Plus a few other similar uses there.
So yes, looks like "Rosetta stone" says that's what it is.