hfss software

why people use the HFSS to simulation Yagi-uda antenna but I haven't seen people using the IE3D to simulate the Yagi-uda antenna , these two soft ware have any difference

Hello frnd,
HFSS is 3-D software, while IE3D is 2.5D simulation software. This much I know.

I have also heard this ...that IE3D is not totally '3d' but 2.5D but what does that mean?

you can build planar circuits on multilayer structure that's 2.5D
about Yagi-uda antenna it will be faster to run it on IE3D if you make the elements planar strips instead of tubes.