Generally, IP cameras will use the main stream for live viewing on a local network or recording to an NVR or CMS/VMS software.  The sub stream is then used for remote access via smartphone tablet where bandwidth is limited.   The substream can also be used to integrate with automation products and touch screens.

To access an Acti IP Camera mjpeg sub stream:

    http://ip:httpport/cgi-bin/cmd/encoder?CHANNEL=2&GET_STREAM

Or to include authentication, assuming username/password is admin/123456 , it would be

    http://ip:httpport/cgi-bin/encoder?USER=admin&PWD=132456&CHANNEL=2&GET_STREAM
 

Make sure B2 frames are disabled.  (under setup, network, RTP)