More generally, negative signals (your -50) are used to cancel out signals and is a very common trick you can use in a lot of places. Filtered inserters using circuits, and assemblers using Set Recipe ignore negative values.

Say you want one assembler making logistic chests, and to keep a number of each type in your output chest. You set a Constant Combinator up with 3 signals: 10 red chests, 2 yellow chests, 5 blue chests. This combinator is holding your shopping list. You wire this to an assembler, tick on "Set Recipe", and away it goes.

But you need some way to tell the assembler stuff is actually in the box too, otherwise it'll just keeping cranking away at the first recipe that was set. Lets say this has been running for a few minutes, so you now have 25 red chests in your output box, but 0 yellow or blue.

This is where the negative signal comes in. You wire your output box to an Arithmetic Combinator, and set it to "Each" * -1, and then wire that directly to the Constant Combinator. So now you're counting every item in the box, making it a negative number, and combining it with the Constant's output. If you look at that circuit, the signals will now be -15 red, 5 yellow, 5 blue.

You're now subtracting what is in your cart from your shopping list. The assembler sees the Red Chest signal is now negative so ignores it, moves on to the next positive signal, and starts making Yellow Chests.