This is a common misconception. With the input we have been discussing in this thread:

Code:

 PS028,006 [KJ <Cj>] [CM< <Pr>] [QWL TXNWNJ- <Ob>]

if that were the default ERE used for separating fields, the default first field would be the empty string before the space at the start of the line. But the actual default first field is PS028,006 (with no leading or trailing <space>s).

The actual default FS value is a single <space> character which is a regex that has a special meaning in awk (i.e., it does not have this special meaning in most other utilities). It is the only utility in the standards where <space> has this special meaning in an ERE used as a field separator. In awk, when an entire field separator ERE is a single <space> character, awk is required to skip leading and trailing <blank> and <newline> characters (where a <blank> character is any character in the current Locale's blank character class) and then fields shall be delimited by sets of one or more <blank> or <newline> characters. In the C and POSIX locales, a <blank> is either a <space> character or a <tab> character; in other locales additional characters may also be included in the list of characters in the blank character class (thereby being ignored at the start and end of a record and being treated as additional elements in field separators in other places).