Clark-Fisher model uses percentages employed in each sector to show that many economies move through three stages: primary, when the largest percentage are employed in primary industry; secondary, when the largest percentage are employed in secondary industry and the post-industrial stage, when most people are in tertiary industry.
This change is driven by an increase in productivity per employee. Increased productivity in agriculture frees people to work in manufacturing. Increased manufacturing productivity and increased income means people spend proportionally less on agricultural goods than manufactured goods, and in turn less on manufactured goods than services.
The model ignores the international economic context - it does not take into account imports of manufactured goods, or the relocation of some countries manufacturing to less economically developed countries.