DEM: (D)igital (E)levation (M)ap.
It is a format that encodes terrain height. Surveyors use it.
Bryce can import them and make terrains from real world measurements.
There are many other formats; but you can convert them to a usable format (.dem or .ddf) for Bryce to use.

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For the OP,
I downloaded some SDTS files to try out:
SDTS is a series of .ddf files. Bryce can use those without conversion. Problem is when you use them there are many terrains created (one for each .ddf in the series).
If you convert the .ddf series to .dem format, then there is just one terrain.

Friends don't let friends use booleans.