Documentation for the ASCII text file format can be found under different names from these sources:

​1)
NIMA:
MIL-STD-2301A
www.nima.mil

2) ISO:
INCITS/ISO/IEC 8632-4-199
http://webstore.ansi.org/default.aspx​

3) Download from:
http://www.directory.net/Science/Reference/Standards/