I'm not well-acquainted with web3j.quorum, so possibly a kind spirit will chime in if this is off. If I'm not mistaken, those would be the enode
addresses for the nodes.
In Ethereum and Quorom, nodes have an "enode address" which is a unique identifier. Have a look at this:
[
"enode://ac6b1096ca56b9f6d004b779ae3728bf83f8e22453404cc3cef16a3d9b96608bc67c4b30db88e0a5a6c6390213f7acbe1153ff6d23ce57380104288ae19373ef@127.0.0.1:21000?discport=0&raftport=50401",
"enode://0ba6b9f606a43a95edc6247cdb1c1e105145817be7bcafd6b2c0ba15d58145f0dc1a194f70ba73cd6f4cdd6864edc7687f311254c7555cc32e4d45aeb1b80416@127.0.0.1:21001?discport=0&raftport=50402",
"enode://579f786d4e2830bbcc02815a27e8a9bacccc9605df4dc6f20bcc1a6eb391e7225fff7cb83e5b4ecd1f3a94d8b733803f2f66b7e871961e7b029e22c155c3a778@127.0.0.1:21002?discport=0&raftport=50403",
"enode://3d9ca5956b38557aba991e31cf510d4df641dce9cc26bfeb7de082f0c07abb6ede3a58410c8f249dabeecee4ad3979929ac4c7c496ad20b8cfdd061b7401b4f5@127.0.0.1:21003?discport=0&raftport=50404",
"enode://3701f007bfa4cb26512d7df18e6bbd202e8484a6e11d387af6e482b525fa25542d46ff9c99db87bd419b980c24a086117a397f6d8f88e74351b41693880ea0cb@127.0.0.1:21004?discport=0&raftport=50405",
"enode://eacaa74c4b0e7a9e12d2fe5fee6595eda841d6d992c35dbbcc50fcee4aa86dfbbdeff7dc7e72c2305d5a62257f82737a8cffc80474c15c611c037f52db1a3a7b@127.0.0.1:21005?discport=0&raftport=50406",
"enode://239c1f044a2b03b6c4713109af036b775c5418fe4ca63b04b1ce00124af00ddab7cc088fc46020cdc783b6207efe624551be4c06a994993d8d70f684688fb7cf@127.0.0.1:21006?discport=0&raftport=50407"
]
From the 7 nodes example: https://github.com/jpmorganchase/quorum-examples/blob/master/examples/7nodes/permissioned-nodes.json
You can see that's describing a network topology where the nodes are known by these identifiers. As a side note, the same concept exists in Ethereum. You can see it in network discovery, the bootstrap nodes, admin.addPeer(), and similar topics.
Hope it helps.