This is Aaron David Fairbanks Bongard problem n. 41. Try to write your solution (with a spoiler if you want). I'll give my solution under a spoiler tomorrow (one more to go before going back to my problems).
For more info about Bongard problems in general take a look at my first messages:
https://mathstodon.xyz/@leonardom/116110015131667314
https://mathstodon.xyz/@leonardom/116110093951382315
This was sufficiently related to the precedent problem, still about basic Graph properties.
Solution to Aaron BP41: the left boxes contain representations of graphs where all edges are part of one Eulerian cycle.
According to Wikipedia an Eulerian trail is a trail in a finite graph that visits every edge exactly once, allowing for revisiting vertices. An Eulerian cycle is an Eulerian trail that starts and ends on the same node. [... A] necessary condition for the existence of Eulerian circuits is that all vertices in the graph have an even degree.
leonardo m.