Back when the NJ Parks Police switched to the 800 NJSP SmartZone system, a decision was made to have one Statewide Parks Police Dispatch and one State Parks Police Ops talkgroup.
Fast forward to 2016 era when the DEP signed an agreement with the State to switch over to the NJICS, additional talkgroups were added for the various divisions, and with Parks, a North, Central, and South talkgroup was added. This was supposed to supplement the Parks 5 VHF channel. However, with the cost of radios the price that they are, and with a very minimal budget, this has not occurred.
Regarding the Parks Police division, Ops 2 & Ops 3 was added. The reasoning for that was based on large scale events at Liberty State Park and if there were any other large events at other parks that needed a separate talkgroup. Also, Ops 2 & Ops 3 were set for 100% secure mode at all times in the event that the Parks PD needed a secure channel to communicate (alarm codes for buildings, criminal subjects with scanners, etc.). With all that goes on at the Trenton dispatch facility, this set-up has worked well for them. Also, different park police officers patrol different parks everyday within their Zone/District. Prior to 2008, each officer was assigned a park and they pretty much stayed in that park/area. This protocol changed in 2008 to accommodate roving patrols and manpower issues in some parks/areas.
I know all of this since I was an employee/affiliate of the State Park Service from 2000-2019. Some things may have changed from 2020 to current, but from my monitoring, not much has changed except for staff using phones to communicate more since radios are not always available to staff. I was also the one who verified all of the talkgroups with the Motorola APX7000 using Unitrunker to confirm them from a Parks Police issued radio.
I hope this helps you out with monitoring them, and the reasoning behind the decisions as to why IBSP doesn't have their own assigned talkgroup.