Remember 'septillion' differs in the US and the UK; it has 24 zeros in the US and 42 in the UK. A US septillion and a UK septillion are not the same number.
Pro Tip 2/3
Hyperbole
Use 'septillion' to express something that's huge or exaggerated, often not meant to be taken literally. I have a septillion things to do today.
Pro Tip 3/3
Abstract Concept
A septillion is used to describe theoretical or cosmic numbers, beyond regular counting or quantities. The chances of winning the lottery are one in a septillion.