T.1 Specifying material definition
When selecting the steel sections, you kept the Material option checked. Consequently, the material definitions were assigned to the members along with the sections.
The STAAD input file commands generated are:
DEFINE MATERIAL START
ISOTROPIC STEEL
E 4.176e+006
POISSON 0.3
DENSITY 0.489024
ALPHA 6e-006
DAMP 0.03
TYPE STEEL
STRENGTH FY 5184 FU 8352 RY 1.5 RT 1.2
END DEFINE MATERIAL
...
CONSTANTS
MATERIAL STEEL ALL