Explanation

Box 1: One-to-many
A customer can have many invoices within one month.
Box 2: Single
For One-to-many relationships, the cross filter direction is always from the "one" side, and optionally from the
"many" side (bi-directional). For
Single cross filter direction means "single direction", and Both means "both directions". A relationship that filters in both directions is commonly described as bi-directional.
Reference:
https://docs.microsoft.com/en-us/power-bi/transform-model/desktop-relationships-understand