When it comes to the capillary-tube kind, the heat is sent to the larger mass of flowing fluid via a small capillary tube that is heated. Both types share the principles of operation which is thermal in nature, they are however so significantly different that there is a requirement for two separate standards.
On top of that, their applications are very different. Commonly a thermal dispersion flow meter will be used for general industrial gas flow, such as in ducts and pipes; capillary tubes on the other hand, are mainly used for smaller flows of gases or liquids that are clean in tunes. In industry, this kind is most commonly used for thermal mass flow meters. However, the subject of discussion here is not the capillary type.