Although this is not an official explanation of the syntax, one example I found shows the following syntax for the vpwlf voltage source file:
The time-value pairs need to be written in floating point, scientific notation or integer number formats (i.e. no scale factors [or units] like "n" or "u"). I believe multiple spaces or tabs are allowed between the entries on each line.
This information was found under the Index topic
"piecewise linear (PWL) vector values"