what is the difference between APPI to IPPI?

IPPI is the FAI international standard licence equivalent for your national authority/federation issued licence.
Only a national license is a properly legally authorised licence.
An APPI card holds no official status in most major countries and is at best a second class certificate issued by a self administering group who tend to teach in countries with no formal or legal licensing systems present.
A legally recognised school from major countries could easily operate in those jurisdictions, and many do (eg BHPA, DHV, USHGPA, SAFA) and offer formally recognised licenses.
APPI is a way for people to teach in those countries without going through the standards required to set up, operate a school and issue formal national licenses.
In summary: A national license and associated IPPI card is a legally formal license; An APPI card is basically an informal statement of attainment.

APPI is an international non-profit organisation active in about 100 countries sharing knowledge and poi’s for paraglider.

IPPI is some kind of international pilot license you have to carry with your national one when you want to fly outside your country and the level on the IPPI card shows what you are allowed to do in other countries.