Answer:

Cell limits apply per job, but not per time. This means if your scheduled job flattens 1M cells a trillion times, over any period of time, you will not be exceeding the 10M cell limit. On the other hand, if your job flattens 10.2M cells one time, you will exceed your 10M cell limit and have to upgrade to a higher plan. To see how many cells your job is flattening, check your Logs.

Cell limits apply only at the individual job-level. If you have 10 different scheduled jobs, which, combined, flatten 90M cells, you are not exceeding the 10M cell limit as long as each of the 10 jobs are flattening fewer than 10M cells.

Here's a helpful article about cells:
​https://www.lifewire.com/what-is-a-spreadsheet-cell-4178208
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In the context of a CSV file a cell is a unit of text between two commas.