What's the default order of Linux sort?

Looks like you are using a non-POSIX locale.

Try:

export LC_ALL=C

and then sort.

info sort clearly says:

(1) If you use a non-POSIX locale (e.g., by setting `LC_ALL' to `en_US'), then `sort' may produce output that is sorted differently than you're accustomed to. In that case, set the `LC_ALL' environment variable to `C'. Note that setting only `LC_COLLATE' has two problems. First, it is ineffective if `LC_ALL' is also set. Second, it has undefined behavior if `LC_CTYPE' (or `LANG', if `LC_CTYPE' is unset) is set to an incompatible value. For example, you get undefined behavior if `LC_CTYPE' is `ja_JP.PCK' but `LC_COLLATE' is `en_US.UTF-8'.

As man sort says, “dictionary-order” means “consider only blanks and alphanumeric characters”.  For example, given the data

The
!quick
brown
@fox
jumps
#over
17
$lazy
  dogs
%42
times.

the unadorned sort command produces

  dogs
!quick
#over
$lazy
%42
@fox
17
brown
jumps
The
times.

(putting the lines that begin with the space characters and the !, #, $, %, and @ symbols¹ ahead of the lines that begin with letters and numbers; i.e., alphanumeric characters), but sort -d produces

  dogs
17
%42
brown
@fox
jumps
$lazy
#over
!quick
The
times.

  dogs is still first, because it begins with spaces, but the special (punctuation) characters are ignored.  17 comes before 42, and fox comes between brown and jumps, in spite of the fact that 42 and fox have characters in front of them that would normally move them before the 17.
____________
¹ in order of their ASCII values: space=040, !=041, #=043, $=044, %=045, and @=0100.  Note that (disregarding the space bar) this is approximately left-to-right order on some keyboards.

To determine the sort order, simply create a file with a different character on each line and the sort it. The resulting output will tell you the sort order.