Cash Registers - General Information

Types of Cash Registers and Where to use them:
Ambitions Register - Consignment Store
Pets Register - Pet Store
World Adventures - Book Store, Food, Nectary, Relics and General Store
Supernatural - Elixir Store Consignment Register
University Life - Roentgen Register for the Nerd Store

Other Registers were added with Premium Content from the Store:
Savvy Seller Register - Allows you to build your own business
Bakery Register
Toy Store Register

The Registers can be found in Build Mode; on the upper right side of the build tab, click on Community.
Each register spawns a Sim to work at the register. The Registers can be placed vacation spots and home neighborhoods. They will only sell items relative to the type of register.
Most Registers will only work on a designated community lot.
The bookstore register and food register from WA work on other community lots - no need for a bookstore or grocery store rabbithole at all. For example, it is possible to place a bookstore register in the library, or a food register at a pool, beach or hangout.

There is a good mod out there offering enhancements to the Savvy Seller Register:
Check out Ani's Savvier Seller here at NRaas. You must own the Store's Savvy Seller Register that came with Midnight Hollow to use this mod.