noun-modifying (irregular)
systematic; structured; organized
Describes knowledge, information, or a body of work that is coherently organized into a system. Used prenominally (before a noun).
彼は体系だった知識を持っている。
He has systematic knowledge.
この本は体系だった情報を提供する。
This book provides structured information.
体系的 is a na-adjective meaning 'systematic' and can be used more broadly as a modifier or predicate, while 体系だった is a prenominal form emphasizing that something has been organized into a system.
From 体系 (system) + 立つ (to stand, to be established) in its past tense form だった, used as a prenominal modifier. The literal sense is 'having stood as a system', i.e., 'systematized'.