Mxsmanic wrote:

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.

No, they can't. Creating a probited area requires administrative action and
publishing on the federal register, which takes time.
Of course temp. flight restirctions can be created at any time.