It is somewhat "complicated".
There is a really useful page on the UNISON website here
"The Labour Party is holding an election for the leadership of the party.
To register to vote, UNISON members must:
◾Be a member of the union before 12 January 2016
◾Be a member of the affiliated political fund (also known as "Labour Link") This is an opt-in when you join where you agree part of your subscriptions can go into the fund.
◾Register with the Labour party as an affiliated union member. When you register you agree to: ◾support the aims and values of the Labour party and that you are not a supporter of any other political party
◾Be on the electoral register at the address given to the union and the Labour party.
The Labour party will be conducting the election. Queries about the ballot or members wishing to check if their applications have been accepted by the Labour Party should contact the Labour Party
UNISON’s nomination process UNISON Labour Link, which represents those members who pay into the affiliated political fund, will follow standard procedures for agreeing any nomination. This will mean:
◾that all candidates have an opportunity to outline their policy agenda and leadership credentials; ◾regional Labour Link committees are consulted;
◾the UNISON Labour Link Committee, consisting of 23 elected members, agrees which candidate to support.
Follow us on Twitter – @UNISONpolitics"
So if you are a UNISON member and got a Leadership ballot party last year (and nothing has changed) you should be okay.
You can check to see if you are in the affiliated political fund (also known as "Labour Link") by logging into your own membership account via MYUNISON (you will need your membership number).
If you are an affiliated political fund (also known as "Labour Link") member you must register with the Labour Party as an affiliated union member by Monday 8 August to get a vote.
It is too late to join UNISON to get a vote and too late if you are in UNISON but not already an Affiliated member.
I hope this is clear to everyone (as mud)