Sonar Core is the library and modding framework behind Calculator, Practical Logistics and Flux Networks, so those mods need it installed to run. It's available for Forge on Minecraft 1.9.4 - 1.12.2.
Under the hood it handles universal energy, item and fluid transfer for easy cross-mod integration, and it also ships a few decorative blocks familiar from Calculator.
Running Flux Networks on a modded Minecraft server? Tick Sonar Core in the panel and it installs alongside it in one click.