Ajaz,
There are 2 ways for configuring or letting the SRX know if the BGP adjacency is external or internal. Actually as you said, you could bring both EBGP and IBGP adjacencies without using the type command (I recreated it in a lab)
*The SRX will always need to have AS configured and just by using the peer-as command under the neighbors config, the device can tell if it is supposed to create an IBGP/EBGP connection. If the peer-as contains an AS number different than the local AS of the SRX, the firewall knows that it is EBGP. Now if the peer-as contains the same AS number it knows an IBGP neighborship will be established.
*You can also use the type internal command to tell the SRX it will be an IBGP neighbor, if you use the type internal you can skip configuring the peer-as, since the SRX will assume it is the same as its local. If you configure type external (which is the default setting), then a peer-as is mandatory.
In summary, with and without the 'type' command are just two ways of accomplishing the same.
