The definition of the FITS binary table extension format does not provide a simple way to specify that a character column contains an array of fixed-length strings. To support this feature, FITSIO uses a local convention for the format of the TFORMn keyword value of the form 'rAw' where 'r' is an integer specifying the total width in characters of the column, and 'w' is an integer specifying the (fixed) length of an individual unit string within the vector. For example, TFORM1 = '120A10' would indicate that the binary table column is 120 characters wide and consists of 12 10-character length strings. This convention is recognized by the FITSIO subroutines that read or write strings in binary tables. The Binary Table definition document specifies that other optional characters may follow the datatype code in the TFORM keyword, so this local convention is in compliance with the FITS standard, although other FITS readers are not required to recognize this convention.
The Binary Table definition document that was approved by the IAU in 1994 contains an appendix describing an alternate convention for specifying arrays of fixed or variable length strings in a binary table character column (with the form 'rA:SSTRw/nnn)'. This appendix was not officially voted on by the IAU and hence is still provisional. FITSIO does not currently support this proposal.