Information from the article by Legalservise lawyers, the full text can be read here -
Invitation to Ukraine for a foreigner - is a special kind of permit to enter Ukraine. More precisely, this is the document that will help you get a visa to Ukraine, if it is required for citizens of your country. This invitation can be issued to you by relatives or friends living in Ukraine - but in this case you can only count on a short-term stay in the country.
But an invitation can be issued by a legal entity as well. Most often this is used in case a foreigner is invited for work in Ukraine and needs to get a visa for crossing the border and further obtaining a work permit or even residence permit in Ukraine.
Invitation to Ukraine will have a limited validity period - it will enable you to apply for a visa. But it does not always guarantee 100% border crossing or further successful execution of documents for legalization of stay in Ukraine - here you need to choose very carefully the person who will send you an invitation and will deal with your registration in the country.
The main difference between a business invitation and, let's say, an invitation issued by your acquaintance in Ukraine is a conditional level of trust that such an invitation will cause in the visa center. A physical person does not take responsibility for your stay in Ukraine, but a legal entity does.
More information in the article.