The Biot number is what we calculate first to know if the lumped system analysis can be applied.
The Biot number is nothing but the ratio of convection at the surface of the body to conduction within the body.
Lc = characteristic length = Volume/surface area
The lumped system analysis can be applied only when the temperature within the body is uniform throughout. This condition is satisfied only when the internal thermal resistance within the body is very low.
Hence the Biot number should be as small as possible.
It is generally accepted that lumped system analysis is applicable when the Biot number is less than or equal to 0.1