The format of the files ERT writes for the forward model. 
The alternatives are:
<dl>
<dt>ASCII
<dd>A text file with formatted numbers.
<dt>ASCII_TEMPLATE
<dd>A plain text file with formatted numbers, and an arbitrary header/footer.
<dt>BINARY_FLOAT
<dd>A vector of binary float numbers.
<dt>BINARY_DOUBLE
<dd>A vector of binary double numbers.
</dl>