Thanks to Topemp who answered some of my questions re. how entries work with this (thought it might be useful to put this as Sticky - anyone know how?):

This is on ITF FAQ section.

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 (A) 9 - When tournament Withdrawal Deadlines pass, how are the Acceptance Lists compiled and will I be committed to play if accepted into a draw?

If a player is accepted into a Main Draw or Qualifying Draw of more than one (1) tournament in a given week, they will be removed from all but one (1) list based on the following criteria
1. Main Draw will take priority over Qualifying Draw
2. Tournament Entry Priority as specified by player at time of submitting entries
If no Entry Priority was specified, then the following criteria will be used
1. Tournaments offering highest prize money, or if equal;
2. The Entry List in which the player was placed highest, or if equal;
3. The tournament which is geographically closest to the players country of nationality

Players should be aware that it is therefore possible that they will be accepted into any tournament they have chosen to enter and will be committed to show up and play in that tournament regardless of players specified priority.

Any player who is accepted into a Main Draw or Qualifying Draw of any tournament is then committed to play that tournament and CANNOT withdraw to play in any other tournament during the same week

Players should only choose to submit an entry into tournaments that they are prepared to go to and play.

return to questions list

More detailed rules here, http://www.itftennis.com/media/204954/204954.pdf go to page 13 for entry and withdrawal info.

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(I believe I am right in saying) players CAN enter more than 1 event, and the player sets the priority their entry should be taken in, but the minute they are accepted, then they are committed, they can withdraw up to a certain point but they can't withdraw to enter a different tournament only to not play at all. After the w/d deadline they are committed and can't withdraw without permission without being fined.

The entry deadlines are 2 weeks for futures, 3 weeks for Challengers and 6 weeks for ATP/ GS.

You can keep up to date with who has withdrawn on http://www.menstennisforums.com/forumindex.php#upcoming-tournaments or http://www.menstennisforums.com/19-challenger-series/ though there may be an official site which I am not aware of.

The Optimist added: That's certainly right for the women up to 100K level and the men up to 15K level, thereafter the WTA and ATP respectively take over the administration and I believe their rules are slightly different. 

Thanks to you too The Optimist!