readable
<code>
DIMENSION [text]
number of dimension\
NODE
number of nodes
0.0 0.0 [x1-coordinate, x2-coordinate, ...]
1.0 0.0
...
ELEMENT
number of elements 
4 1 1 2 3 4 [element type, property type, node number 1, node number 2, ...]
4 3 1 4 5 6
...
BOUNDARY CONDITION
number of boundary condition [only homogenous Dirichlet condition] 
2 1 [node number, direction number]
6 2
...
FORCE
number of force [only non-zero force]
3 2 -2.0 [node number, direction number]
4 2 -2.0
...
PHYSICAL PROPERTY
1 1.0 1.0 [property number, physical properties...]
...
OTHERS [parameters for calculations...]
</code>

For element type, refer to page 9 in www.vtk.org/VTK/img/file-formats.pdf
<code>
ex.
triangle = 5
polygon = 7
quad = 9
tetra = 10
hexahedron = 12
</code>