Analysis of information sources in references of the Wikipedia article "Satellite Catalog Number" in English language version.
Q: What criteria are used to determine whether an orbiting object should receive a catalogue number and International Designation? A: We must be able to determine who it belongs to, what launch it correlates to, and the object must be able to be maintained (tracked well).
10 centimeter diameter or "softball size" is the typical minimum size object that current sensors can track and 18 SPCS maintains in the catalog.