Guilliman made a few changes to the Codex Astartes when he came back. One of these was that (non-scout) brothers from companies 6 to 10 can temporarily be placed into one of the main battle companies (for example, 2nd) to bolster their numbers above 100. So, in 2nd company, you can have a bunch of Hellblasters and devastators added in from 9th company, and a bunch of Vanguard marines added in from 10th company, etc. When these brothers temporarily join a battle company, they take on the shoulder trim of that same company (in the case of Ultramarines 2nd company, it's gold trim). So you could theoretically have 500 space marines all wearing 2nd company trim and being in that company for an entire campaign. And like others have said, Primaris marines have training in other roles, so your intercessors might swap out their gear to become infiltrators. Mark 10 armour is very modular, so it can be fitted with Gravis or Phobos parts prior to battle.
Long story short, battle companies are no longer set in stone. They're constantly in flux, taking on additional brothers or changing the load-outs of their existing squads to fit a campaign's needs. I think GW did this so that people can make collections of space marines that have more than 100 marines, but all wearing the same shoulder trim. You could also have more than 6 battle-line squads (for example, you could have 7 ten-man intercessor squads in a company), and it will still be fitting of the new company structures that Guilliman added to the codex.
Edited by Tamiel, 07 April 2019 - 05:24 AM.