Under "Top Two," political parties do not nominate candidates at all. Instead, all candidates, regardless of party affiliation, run on the same primary ballot. The top two vote-getters then face off in the general election, again regardless of party affiliation. This means that the general election always has exactly two candidates on the ballot, and those two candidates may be from different political parties or from the same political party.
California and Washington use Top Two for state and congressional offices. Nebraska uses a Top Two system but without party labels on the ballot at all for the election of its nonpartisan state legislature.
Learn more here: How Top Two Compared to Ranked Choice Voting
The Louisiana system, sometimes called the "Cajun Primary," eliminates the primary election altogether. Instead, all candidates, regardless of party affiliation, run on the same ballot in November. If a candidate receives more than half of the votes, that candidate is elected. If no candidate wins with a majority, the top two vote-getters face off in a December runoff election. Qualified absentee voters receive a ballot for the November election and a ranked ballot for the December runoff, so that they can vote as normal in the general election and then have their ranked ballot count for whichever runoff candidate they ranked highest in the runoff election.
Although Louisiana law refers to the election in November as the "primary" and the December runoff as the "general" election, the November election takes place on the federally mandated Election Day and most candidates win office by receiving a majority vote in that election, so it is best understood as a general election, with the December election as a contingent runoff.
The Louisiana system is sometimes mistakenly equated with the Top Two system, but holding the first election in November and electing any candidate with more than 50% of the vote in that election makes it sufficiently distinct that it should not be understood as a mere variant of Top Two.
Alaska uses a variant on Top Two which allows four candidates to advance to the general election instead of two. This gives voters more choices during the general election when turnout is typically highest, reduces the chance that one major party will be shut out of the general election, and improves the chances for a minor-party or independent candidate to appear in the general election.
In Alaska, the general election is decided by ranked choice voting among the four candidates.