It should be set in VDEC_BUF_REQUEST_S parameter of Vdec_requestBitstreamBuffer API.You should populate the correct channel ID when requesting bitstream buffers
Refer /dvr_rdk/demos/mcfw_api_demos/mcfw_demo/demo_vdec_vdis_bits_rd.c
You seem to be using an old release . Pls migrate to DVR RDK 4.1 as older releases like RDK 3.5 are not supported.