Requests ingredients and crafts items one at a time, with the recipe and amounts easily adjustable by the player. Good for low-volume, expensive items like nuclear reactors and rocket silos.
To use: set the recipe in the assembler, then toggle the constant combinators to increase or decrease the craft count (the "Toggle Entity" keybinding is convenient here). The values in the combinators can also be adjusted to add/subtract more than 1 at a time. The current count is shown on the display panels if it's less than 99.
Original idea based on https://www.reddit.com/r/factorio/comments/6l9ji9/allow_requester_chests_to_set_requests_and_read/, but updated to take advantage of the improved combinators in 2.0.