What is a relationship groups in Neo4j?

Hi there

I was reading about the storage limits avaialbe in 3 formats in the Neo4j operation manuel.
As you can read in the limits table, there are the following limits in Neo4j by default:

|Property keys|2^24 (16 777 216)|
|Nodes|2^35 (34 359 738 368)|
|Relationships|2^35 (34 359 738 368)|
|Properties|2^36 (68 719 476 736)|
|Labels|2^32 (4 294 967 296)|
|Relationship types|2^16 (65 536)|
|Relationship groups|2^35 (34 359 738 368)|

They all make sense to me, since they are all based on a concept that we can learn from graph academy or the manuel, except one, what is a relationship groups ?