Order of elements
Definition
Flux provides for the user two types of finite elements: first order elements or second order elements.
Type of element  Location of nodes  Interpolation function  

Automatic mesh generator  Mapped mesh generator  
First order 
Linear (1st order polynomial) 

Second order 
Quadratic (2nd order polynomial) 
With second order elements, the number of nodes is far larger than with first order ones but accuracy is largely increased.
2D application
For a 2D application, the mesh generation is completed with meshing the faces, and second order face elements are directly generated. Remark: first order mesh is also available (even if not really suggested to users) for all the 2D applications except the axisymmetric domain type.
3D application
In 3D, meshing the volumes leads to first order volume elements.
In several physical configurations, it is possible to solve with first order elements.
Generally, it is better to generate the second order elements.
Generate second order elements (3D)
To generate second order volume elements:
Step  Action 

1 
Activate the command:

→ 
Elements of second order are generated. The number of mesh elements is increased. 
Come back to first order elements (3D)
To come back to first order volume elements:
Step  Action 

1 
Activate the command:

→  Mesh elements are the first order elements. 