Because Calamondin are not very well known in Europe, some gardeners mistake them for Kumquats, as both are small citrus fruit trees with orange fruit, but the two are very different. Calamondins are not kumquats - kumquats have a different internal structure (only 4 segments) and thicker peel, while Calamondins are more like miniature oranges, and have seeds internally.
Importantly to cooks, the flavour is completely different. Kumquats are sweet while Calamondins are exceptionally sour. If you use a Calamondin in a recipe, the effect is closer to using a whole full size lemon.
Calamondin can be used to cook while still green, but the flavour will be even sharper. When orange, they are comparable to lemons. When green, it is closer to limes, and the peel is far less sweet.