String types that hold bmp-chars.
bmp-string is a string that can hold bmp-chars, that is characters with code below
#x10000 (that is 16-bit). This corresponds to the Basic Multilingual Plane of Unicode.
simple-bmp-string is the simple version of
bmp-string, that is it is a simple-array of bmp-chars.
bmp-string was new in LispWorks 7.0. In LispWorks 6.1 and earlier versions text-string is similar to
bmp-string. However, in most cases where you use text-string you probably still want to use text-string (using its new meaning, covering all the Unicode range).
LispWorks User Guide and Reference Manual - 20 Sep 2017