The FMX generates a Multiplane.txt file when you are doing a topographic map as well as the shape files for Surface.
The .txt file is meant to go into Trimble's Multiplane program for land leveling design. It is a point list with the first point being the Master Benchmark Offsets, GPS coordinates and elevation. The rest of the data is point data with point number followed by X, Y and Z offsets from the Master Benchmark. B is for border point.
Example:
0001 5000.000 5000.000 100.000 MB N46:52:32.015 / W96:51:22.380 0.000
2 5001.28 5000.30 99.970401 B
3 5004.23 4998.98 100.207556 B
4 5006.84 4996.67 100.150267 B
5 5008.47 4993.70 100.039718 B
6 5009.19 4990.75 100.065499 B
7 5009.97 4987.20 100.024815 B
8 5010.27 4984.05 99.9851890 B
I think the original poster may have Multiplane topographic text files from the older FMX or GeoStar system and would like to be able to import that into Surface for drainage design.