The COSC mainly works on the observation of movements, but also tests watch heads. ISO 3159 specifies that certification can be carried out at watch head or at movement level, the title "chronometer" applying in the same way in both cases.
The choice therefore lies with the manufacturer. The testing of the watch head is the most absolute, since there is no further intervention at production level. But testing of the watch head is more expensive, since the setting up and observation work, involving the presence and manual work of an operator for each part, takes longer. This work is mainly carried out on so-called exceptional watches.