The ATC system is divided into 2 levels: low altitude sectors from ground level to 18 kft. MSL , and high altitude sectors from 18 kft. MSL to 60 kft. MSL, and every sector has a different set of controllers which may be located in a different physical location.
A basic rule of the ATC system is maintaining a vertical separation of 2 kft. vertical and 3 nm horizontal between air vehicles. If you request a waiver of 16 kft. MSL or lower, the high level ATC has no need to know of your activities and life is easier as you normally only have to deal with one ATC control center. That ATC will issue a NOTAM for your activities and vector all IFR flights around your location. Once you need to exceed 16 kft MSL, the high altitude ATC sector controller must also get involved. This is the high speed air travel corridor where airliners typically travel at 600 mph (10 miles per minute) so ATC must look out over 250,000 square miles to see what might be traveling thru your section of the airspace within the next 30 minutes. This is the reason why high altitude launch windows are typically limited to 20 minute windows for flight clearances over 16 kft. It takes several minutes for the sector controller to determine which flights will be influenced by your launch activities and will have to be diverted around it, and then it will take several more minutes to redirect those flights. As the controllers sight line is only a few hundred miles, he most likely does not know what's coming into his control region more than 30 minutes ahead of time so that determines your call-in waiver window.
For Potter (~1 kft MSL), the low level ATC sector is Rochester and the high level ATC sector is Cleveland. Rochester continually maintains horizontal AC separation during LDRS operating hours for launches to 15 kft AGL, and Cleveland will maintain high latitude separation for defined 20 minute launch windows on request during LDRS for launches between 15 kft. to 23 kft. AGL.
Bob