Standard segments can be turned into Persistent Segments via this button on the "Update Partners" page of the segment:
Users are collected beyond the standard 90 day lookback: once a device is added to a Persistent Segment, the associated KUID is never removed. This segment type is typically used to track seasonal traffic for subsequent analytical or targeting initiatives.
Please be aware that all historic users will be lost if the persistent segment is edited, and unlike standard segments, the persistent segment type will not show a last recomputed date.
Please also note that user that delete their cookies will still be counted for the segment population, but can not be reached via activation. So the actually usable part of the segment population is lower for persistent segments than for other segment types.
Even though you are able to select recency in the Salesforce Audience Studio UI when building a persistent segment, recency is not used in persistent segments. Frequency is still accounted for. If your account has a default lookback window of 90 days, all persistent segments will default to a 90 day lookback window, even if recency is specified as 30 days or 60 days, or any other time frame.
Note: If you have the extended lookback feature enabled, say a 400 day lookback, persistent segments will use a 180 day lookback. For example, if you select a frequency of 2 and a recency of 30 days, the persistent segment will be built for any device that has met the rule(s) twice in the last 180 days.
If you build a persistent segment with an event pixel that was placed beyond the last 90 days (or 180 days if you have the extended lookback feature), the segment will include all devices that event pixel has ever seen, from the first day it started firing, even if it was more than 90 days ago (or 180 for extended lookback customers).
Persistent segments are not available in all Audience Studio license packages.