When declarer, you should always name the suit and denomination of the card you want
to play from dummy. If you don't, you may find that the defence are allowed to
make assumptions and you may not be able rectify it even if it was "obvious" that you
meant something else.
If you say "play anything", or words to that effect, then either defender may choose
a card for you to play.
If you only specify the suit, or say something like "low", you are deemed to have
called for the lowest card of the suit.
If, when leading, you only name the denomination then you are deemed to be continuing
the suit in which you won the preceding trick (assuming you have a card of the
named denomination in that suit).
When not leading, if you only name the denomination then you must play a
card of that denomination if you can legally do so. If you have two or more
then you can choose which.
If you say something like "win the trick", dummy should play the lowest winning
card.
If you call "high" then you are normally deemed to have played the highest card
in the suit. However, if it's the last card in the trick then it's the same as
saying "win the trick" - ie. the lowest winning card should be played.