Tim writes:

About 35 miles southwest of KLSC, Salt Lake City Intl (just beyond the
Mode C veil) there is a box marked by a thick broken magenta line on the
Salt Lake sectional. Nearby, there is a Notice: "For National Security
reasons, pilots are requested to avoid flight at/below 4,000' in this
area."

That's the Tooele Army Depot, where chemical and biological weapons are stored
and destroyed.

The area is around one of the chemical weapons stockpiles and
incinerator facilities in the vast Dugway Proving Ground area.

Yes, see above.

Is this broken magenta line used anywhere else? Does it have an official
meaning?

Yes, it's a National Security Area (NSA). Pilots are requested to voluntarily
avoid NSAs. Many of them replace TFRs, and they are less restrictive than
TFRs. However, NSAs can be instantly transformed into prohibited areas by
NOTAM, so beware.

There are lots of other NSAs. There's a small area like that just southwest of
R-6701 in Seattle for example (between the "wings" of the Chinook MOA), and
another one east of that that's so small it has only one little magenta block.
There's another one just east of Livermore, California. I think several
national mints have blocks like this, too. There's one just east of the
Pueblo VOR southwest of Denver that might be one of these.

If National Security is the issue, why isn't the area marked as
Restricted instead? Perhaps it should say "For reasons of your health
and not breathing toxic chemical weapon waste smoke, you might want to
fly elsewhere?

NSAs are voluntary restrictions that can be made official at a moment's
notice. Apparently the idea is to restrict them less and make them look more
"friendly" while still retaining the option of making them fully prohibited
almost without notice.

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