For example, the Democratic New York primary is a closed primary which means only voters registered as Democrats have the right to vote. However, the Democratic Californian primary is an open primary which allows voters registered as Democrats or as NPP to vote.
Voters can change their party preference at any time but they may be required to be registered under a specific party for a certain period of time in order to be given the right to vote in some States. For instance, in order to vote in New York you needed to be a registered Democrat for the past 6 months prior to the election date. This is a disadvantage to candidates who are well supported by independent voters (NPP) as many of them did not change their registration on time. That said, these rules are those of the party and while they can be considered unfair, this doesn't qualify for election fraud.
An election fraud tactic the report mentions is "registration tampering" which consists in switching the registration of voters without their consent and knowledge in order to suppress their right to vote. There have been numerous reports of registration tampering across most States with voters being switched from Democrats to Republicans or from Democrats to NPP and so on. Registrations seem to have been switch electronically (change in the database) some even involving forged signatures.