Different Contexts
Source GM Core pg. 23
Just because something is common or uncommon in one context doesn't necessarily mean it's the same in others. This is specifically true when comparing the commonality of a creature and an ancestry. For instance, while orocorans are a relatively common foe for adventurers to encounter in the Gelid Edge and are thus a common creature, in most settings they're still far less prevalent than humans or ysoki and would be an uncommon ancestry.
Because uncommon elements are available in certain circumstances, they often vary by locale, even within the same solar system. For instance, weapons wielded by soldiers of the Azlanti Star Empire, such as the aeon rifle might be uncommon in the Pact Worlds, but in the Azlanti Star Empire, an aeon rifle would be common and some Pact Worlds weapons, perhaps the aucturnite chakram, might be uncommon. Similarly, in the Veskarium, uncommon vesk weapons like the doshakari might be common. Some items are uncommon everywhere, such as the quasar solarian crystal.
The same is true of many ancestries and alien creatures. While ijtikri and talphi are common in the Veskarium, they're less often encountered in the Pact Worlds. This is particularly noticeable for ancestries who live primarily on a single planet, such as copaxis, or who lack access to Drift travel. Likewise, khefaks are common creatures on Akiton but are uncommonly encountered on Castrovel and are rarely encountered on Vesk-8, though they've infested many desert worlds, industrial parks, and junkyards near and far. Creatures that are native to a specific place are more likely to be found in other places with similar environments.
Similarly, technology levels can be a factor in rarity. Civilizations with industrial levels of technology probably lack access to starships but likely still have machine guns and shock truncheons, while civilizations with medieval levels of technology might treat all Starfinder items as rare or unique and instead use items from Pathfinder in their place. Conversely, PCs accustomed to Pact Worlds level technology who suddenly find themselves on a technologically advanced world will encounter rare and unique items that are common in this new region. Using rarity in this way can help differentiate technology levels for adventures that occur on isolated planets, alternate realities, or in other time periods.
Because uncommon elements are available in certain circumstances, they often vary by locale, even within the same solar system. For instance, weapons wielded by soldiers of the Azlanti Star Empire, such as the aeon rifle might be uncommon in the Pact Worlds, but in the Azlanti Star Empire, an aeon rifle would be common and some Pact Worlds weapons, perhaps the aucturnite chakram, might be uncommon. Similarly, in the Veskarium, uncommon vesk weapons like the doshakari might be common. Some items are uncommon everywhere, such as the quasar solarian crystal.
The same is true of many ancestries and alien creatures. While ijtikri and talphi are common in the Veskarium, they're less often encountered in the Pact Worlds. This is particularly noticeable for ancestries who live primarily on a single planet, such as copaxis, or who lack access to Drift travel. Likewise, khefaks are common creatures on Akiton but are uncommonly encountered on Castrovel and are rarely encountered on Vesk-8, though they've infested many desert worlds, industrial parks, and junkyards near and far. Creatures that are native to a specific place are more likely to be found in other places with similar environments.
Similarly, technology levels can be a factor in rarity. Civilizations with industrial levels of technology probably lack access to starships but likely still have machine guns and shock truncheons, while civilizations with medieval levels of technology might treat all Starfinder items as rare or unique and instead use items from Pathfinder in their place. Conversely, PCs accustomed to Pact Worlds level technology who suddenly find themselves on a technologically advanced world will encounter rare and unique items that are common in this new region. Using rarity in this way can help differentiate technology levels for adventures that occur on isolated planets, alternate realities, or in other time periods.
Access Entries
Source GM Core pg. 23
Uncommon elements sometimes have an Access entry in their stat block. An Access entry usually speaks to elements of a character's backstory or experiences, such as “worshipper of Ibra,” “member of the Starfinder Society,” or “from Absalom Station.” A character who meets the access requirements can freely choose that option just like they would a common option, even though it's uncommon. Unlike a Prerequisites entry, an Access entry never speaks to mechanical requirements needed to make the rules function, so if you'd like to modify Access requirements, you can do so without worrying about altering game balance.