Entity Reference Question

>From XSLT Programmer Reference, it says

<!DOCTYPE xsl:stylesheet [
     <!ENTITY copy  "&#169;">
     <!ENTITY  nbsp "&#160;">
]>

How can I know that 169 represent copyright and 160 represent non-breaking
space ?

Where can I them up ? Is there any means I can set my own number for my own
string substitution?

Thanks.




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