Plumber question - twin adapter for waste

[SpokeyDokey]

Why does the little twin adapter for my washing machine/dishwasher waste have a small open slot in the top?

Just puzzling me slightly.

 

[Levo-Lon]

So you can connect both..if its the trap adapter..

 

[Tim Hall]

Why does the little twin adapter for my washing machine/dishwasher waste have a small open slot in the top?

Just puzzling me slightly.

It's an airvent, to stop the water in the U bend of the trap being siphoned out.

 

[SpokeyDokey]

It's an airvent, to stop the water in the U bend of the trap being siphoned out.

@Tim Hall

Many thanks.