To reduce the burden on voters, the voters have an option to allow their favorite party to choose the rankings for them. This post explains how you can implement an ATL online election with OpaVote.
Above, is an example of a ballot that includes an ATL option. The thick black horizontal line there is the "line" of above the line. A voter can vote above the line (ATL) or below the line (BTL) but not both (I suppose some implementations may allow both but I'll skip that to simplify the discussion).
An ATL vote is to select a party of the four parties competing in the election. If a voter picks a party, then the voter's vote is determined by the party's "ticket". Before the election, each party specifies its ticket, which corresponds to a ranking of the candidates competing in the election. Each party presumably ranks it own candidates the highest and followed by candidates of ideologically similar parties.
A BTL vote allows the voter to rank the candidates any way he or she wants.