Wikipedia
A PrintableString is a restricted character string type in the ASN.1 notation. It is used to describe data that does only consist of a specific printable subset of the ASCII character set.
According to the ASN.1 Specification of basic notation, the character set of PrintableString can be expressed as:
Name
Graphic
Latin capital letters
A,B, ... Z
Latin small letters
a,b, ... z
Numbers
0,1, ... 9
SPACE
(space)
APOSTROPHE
'
LEFT PARENTHESIS
(
RIGHT PARENTHESIS
)
PLUS SIGN
+
COMMA
,
HYPHEN-MINUS
-
FULL STOP
.
SOLIDUS
/
COLON
:
EQUALS SIGN
=
QUESTION MARK
?
The PrintableString definition does not include the at sign or ampersand (&). This sometimes causes problems for naive implementers who attempt to put an SMTP email address into an X.509 digital certificate Distinguished Name.