The answer can be found in the info directory:
Specify a sort field that consists of the part of the line between
POS1 and POS2 (or the end of the line, if POS2 is omitted),
Each POS has the form `F[.C][OPTS]', where F is the number of the
field to use, and C is the number of the first character from the
beginning of the field. Fields and character positions are
numbered starting with 1; a character position of zero in POS2
indicates the field's last character. If `.C' is omitted from
POS1, it defaults to 1 (the beginning of the field); if omitted
from POS2, it defaults to 0 (the end of the field). OPTS are
ordering options, allowing individual keys to be sorted according
to different rules; see below for details. Keys can span multiple
Example: To sort on the second field, use `--key=2,2' (`-k 2,2').
See below for more examples.
Scripts intended for use on standard hosts should avoid obsolete
syntax and should use `-k' instead. For example, avoid `sort +2',
since it might be interpreted as either `sort ./+2' or `sort -k 3'. If
your script must also run on hosts that support only the obsolete
syntax, it can use a test like `if sort -k 1 </dev/null >/dev/null
2>&1; then ...' to decide which syntax to use.
So... I'd say that it means "leave the 1st field alone" and "leave the last two fields alone", but in a deprecated syntax.