"Optional tombs" refers to burial places or graves that are not necessary or required. This means that people have the choice to be buried there or not, as it is not mandatory. Full definition
Taking the criticism people lobbied at the prior entry in the series, Rise is packed to the brim with content including optional tombs, collectibles and side missions for players to explore and engage with during their travels. (shortpause.com)
Part of the old Tomb Raider spirit also awaits players with optional tombs, hidden areas where the bread and butter of the series is refined with laser precision. (gamesugar.net)
In a nod to the series name there's a total of seven optional tombs for you to find and raid scattered around the island, each of which ditches combat in favor of platforming and puzzling. (wolfsgamingblog.com)