The IsSubSequence(s,t) command returns the value true if s is a subsequence of t and the value false otherwise.

If s is the empty string, the command returns true.

To test whether a string is a substring of another, use StringTools[Search].

All of the StringTools package commands treat strings as (null-terminated) sequences of $8$-bit (ASCII) characters.  Thus, there is no support for multibyte character encodings, such as unicode encodings.

$\mathrm{with}\left(\mathrm{StringTools}\right)\:$

$\mathrm{IsSubSequence}\left(''abc''\,''abc''\right)$

$\mathrm{true}$

$\mathrm{IsSubSequence}\left(''abc''\,''abx''\right)$

$\mathrm{false}$

$\mathrm{IsSubSequence}\left(''abc''\,''axbyc''\right)$

$\mathrm{true}$