Neither. I think the idea is to target austerity and hard Brexit, because supporters of both don't have a voice in Parliament. It's becoming increasingly obvious that there isn't an effective opposition, so the smaller margin between the Tories and the total of other parties could, at least, cause May a few headaches.
I think (again, because I could be wrong) is to prevent May having such an overwhelming majority that she can do what she wants.
One of the reasons (apart from Corbyn) that the Conservative majority in the polling is so huge is that the Conservatives have adopted Ukip policies, as Ukip itself has collapsed.
The LibDems and Labour have refused to form a leftish pact, so they won't stand down candidates to give another party a clear run, so the only alternative if you want a leftish agreement in Parliament is to vote tactically. That doesn't mean that the LDs and Labour will form a coalition, but they could block really hard-right legislation in Parliament by voting against it. The Lords currently has an anti-Tory majority.
The losers with tactical voting are likely to be Conservatives, because there are so many LD/Conservative marginals, where LDs have held the seat in the past and about 20 seats, where there was a Remain majority, but the Conservative MP supported Leave.
An example: If you live in a Plaid/LD marginal and strongly support Remain more than any other issue, it probably doesn't matter how you vote, because both support Remain BUT if there is any chance that the Remain vote is split and the Conservatives sneak in, it would be a good idea for people to decide which one to vote for.