According to this, Simple Identifier is decided with rules below.
A simple identifier is a Unicode character sequence with the following restrictions:
It consists of at least one and at most 128 Unicode characters.
The first character MUST be the underscore character (U+005F) or any character in the Unicode category “Letter (L)” or “Letter number (Nl)”.
The remaining characters MUST be the underscore character (U+005F) or any character in the Unicode category “Letter (L)”, “Letter number (Nl)”, “Decimal number (Nd)”, “Non-spacing mark (Mn)”, “Combining spacing mark (Mc)”, “Connector punctuation (Pc)”, and “Other, format (Cf)”.
Non-normatively speaking it starts with a letter or underscore, followed by at most 127 letters, underscores or digits.
According to this,
Simple Identifier
is decided with rules below.https://docs.oasis-open.org/odata/odata-csdl-xml/v4.01/cs02/odata-csdl-xml-v4.01-cs02.html#sec_SimpleIdentifier