They are organised thematically using themes which are commonly used in the IELTS exam.
The first 6 units are intended to familiarise the student with the basic test types in the exam and introduce each test type in order. Units 7-9 consolidate the test types using more complex content (longer texts, more difficult grammar); The last three units use all the test types into combination activities which are very similar to the actual exam.
The materials under each theme are organised as in the exam and the intention is that students can take a completely flexible approach to how they use the materials. Students can practice a particular skill (reading, writing, etc), a particular test type (m/c, gap flll, etc), or simply work systematically through the materials unit by unit.
The Academic module is aimed at around band 5.0 with the intention of raising the band to a 6.0 if all the materials are covered. The General Training module is aimed a bit lower - around 4.5 to raise to 5.5.