Understanding HEVC HW encoder plugins

Hello

I am trying to use the HEVC hardware accelerated encoder and i found 2 plugins.
1. MFX_PLUGINID_HEVCE_HW
2. MFX_PLUGINID_HEVCE_GACC

When i tried to simply run sample_encode with h264 encoder, the sample encode failed with an error (see attached file 1.txt)

but when i tried to use the hevc_gacc plugin sample_encode ran as expeected (attahced file 2.txt)

I would like to ask what is the difference between the plugins? and when should i use each one of them?

Some details :
OS  : Centos 7.2.1511
CPU : Intel(R) Xeon(R) CPU E3-1275 v5 @ 3.60GHz
GPU : 00:02.0 Display controller [0380]: Intel Corporation Device [8086:191d] (rev 06)    (from lspci)

Thank you
Koby