"Assimilate(d)" is a text-saving mechanic (a la "download") rather than a keyword. When a player assimilates a card, they take command of it. Unless otherwise specified an assimilated card's location does not change.
When a player assimilates a personnel or ship, it becomes an affiliation matching one of their headquarters missions (if they command multiple headquarters, they pick one of the eligible affiliations).
When a player assimilates an event, it is moved it to their core.
When your opponent plays an interrupt, place your assimilated personnel that they own on their ship to prevent that interrupt. They assimilate that personnel.
Order - Assimilate an opponent's personnel present with your assimilated personnel that he or she owns.
The text of these cards is edited to refer to assimilating/assimilated cards, as appropriate.
The Borg-themed cards might also gain an "Assimilation" keyword, as appropriate.
Bio-neural Computer Core
Not Quite Domesticated Pets
Self-Sealing Stem Bolts
Adapted to Service Us
Adding to Our Perfection
Borg Cutting Beam
•Display of Wealth
•One With the Borg
Scum and Villainy
Targeted for Assimilation
•The Perfect Tool
The Will of the Collective
Bait and Switch
Knowledge and Experience
Target of Opportunity
•Grid 532, Assimilate Species 10026
•Wolf 359, Assimilate Resistance
•Bashir Founder, Imperturbable Infiltrator
•Berlinghoff Rasmussen, Temporal "Historian"
•Borg Queen, Perfectionist
•Gegis, Exploitative Profiteer
•Icheb, Genetic Weapon
•Joaquin, Superhuman Lieutenant
•Kruge, Instinctive Commander
•Letant, Sneering Liaison
•Lovok Founder, Effective Changeling
•Rota Sevrin, Deranged Doctor
•T'Jon, Felicium Addict
•U.S.S. Enterprise-E, Royal Refuge
Commodities technically should use the assimilation phrasing but for flavour reasons might be left unchanged.