What does the $startWith argument contain in a $graphLookup stage?

what refers the parameter $base.
Captura de pantalla 2022-05-30 204645

if you look at the $graphLookup documentation, you will see that startsWith: is an expression.

in an expression, $fieldName means take the value of the field named fieldName.

in the case you shown, it means that the graphLookup starts with the value of the field base for each matching documents of the aggregated collection.

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