In a much cited anecdote by Fritz Saxl, Warburg’s long-term academic collaborator and first director of the Warburg Institute, we find Aby “standing tired and distressed, bent over his boxes with a packet of index cards, trying to find for each one the best place within the system”.[6]
[6] Fritz Saxl, The History of Warburg’s Library (1886-1944) cited in Gombrich 1970, p. 329; Marchand 2023, p. 186; Steiner 2013, p. 155; Wedepohl 2014, p. 389.
It's only in a physical card system one might worry a bit about "best place" for a card. Some of it is down to one's future self being able to find it, but cross-references could have been placed or an amanuensis might have created an exact copy for multiple copies in many locations.
What does Ernst Bernheim have to say on the topic of filing?