What is a free DEMO font mean? what are the restrictions and licensing of usage?
can it be used for commercial purpose?
>... can it be used for commercial purpose?
if it is not forbidden, I guess it's allowed.
It depends on the licence. It's going to vary from font to font. Be sure to read it carefully before using it to understand what you can and cannot do with it. Many demo fonts are released with restrictions on commercial usage, but some are not.
Sometimes it means the font is crippled in some way.