Official rule text
63.2 The primary way by which players acquire PDS units is Planets, Readied, Resources, System Tiles, Technology by resolving either the primary or secondary ability of the “Construction” strategy card. 65 PLANETARY SHIELD (UNIT ABILITY)
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Source: Twilight Imperium: Fourth Edition - Living Rules Reference: Prophecy of Kings
Expansion: Prophecy of Kings
Version: 2.0
Reference: Rule 63.2