A variable keeps a value that later actions can read and change. Initialize variable gives it a name, a type and a starting value. Set variable replaces the current value. Increment variable adds a numeric amount, so both the stored value and the increment must be numbers. A source field that looks like a number may need conversion first.
Initialize an accumulator before the loop that updates it. Initializing it again on each iteration either fails or loses the previous total. After the loop, read the variable once to produce the final summary. Empty input should leave its initial value intact. These flows execute local loops sequentially. In workplace flows, shared variable updates need extra care when concurrency is enabled; the visible order of cards alone does not make parallel updates safe.