The replaceable rules are a collection of about 40 sections extracted from the Corporations Act 2001 that give a general framework for running an Australian company.
These rules govern the internal relationships of officers and shareholders, but do not cover more practical rules relating to how a company is run on a day-to-day level.
To read more about constitutions and the replaceable rules and to determine the suitability of both for your company, please read this page on the ASIC website: Constitution and replaceable rules.