The For block always serves as the beginning of a loop. See Loops with the For block. Technically, the For block is a control block used to repeatedly run a series of blocks in your flow. It accepts an array as input, and sends each item in the array from its output port, one item at a time.
At the end of the loop, after whatever blocks you use to process data within the loop, you must have a Collect block. The Collect block gathers the individual items sent from the For block, and places them back into their original array order.
List - Accepts an array of values like [1,2,3] or ["one","two","three"]. The For block requires an array to be sent this input in order to run.
Start - Connecting another block to this input port is optional. If no block is connected here, the arrival of data at the List input port triggers the For block.
If you do connect another block to the Start input, the For block runs only when it has received data from both connected blocks. This provides control over the order in which blocks run in your flow. For example, if you connect an HTTP Request block to the Start input, the For block will run only when data has arrived at both the List and Start inputs.
Item - Sends each item in the list sequentially, one at a time.
To set up the For block, connect the List input port to another block’s output port that sends an array. Blocks that can send an array include the following:
Optionally, you can connect the Start input port to another block’s output port to trigger the For block.
To see the For block in an example flow, check out Flow Snippets: For.
To supply the array that the For block requires as input, use any of the blocks described in the Setup section.
Besides the For block, the other option for beginning a loop is the Repeat block. See Loops with the Repeat block.
For more information about creating loops in Flows and tutorials that use the For block, see the following: