A more thorough explanation:

Financial-core membership is a type of union membership where a private-company employee pays the union's initiation fees and periodic dues but is not a full union member. This type of membership is only allowed in states without a right-to-work law, where a union-security contract clause can require employees to pay financial-core membership dues but cannot require full union membership. The dues are limited to the amount required to support the union's representational activities, such as collective bargaining.

For example, if a private-company employee in a state without a right-to-work law wants to work for a company that has a union-security contract clause, they may be required to pay financial-core membership dues. This means they will not have full union membership rights, but they will still have to pay a portion of the union dues to support the union's representational activities.

Another example is in the public-employment sector, where financial-core membership is also known as fair-share membership or agency-shop membership. This means that public employees who are not full union members may still be required to pay a portion of the union dues to support the union's representational activities.