WTE - Whole Time Equivalent
An WTE of 1.0 means that the person is equivalent to a full-time worker, while an WTE of 0.5 signals that the worker is half-time etc.
WTE should be what you are contractually funded for. This should be what you should have. The total establishment being all roles fully staffed, including those currently unfilled. If number of individuals 0 then WTE must also be 0. Unless the contract changes, this should not vary total should not vary from day to day.
If you are funded to have 1.5 WTEs for a band 6 nurse, you may have one person working full time (1 WTE) and one person working half a day (0.5 WTE). Together they make up the 1.5 WTE that you are contractually funded to have.
If for example your part time person resigns or there is no one currently in post at the moment, your total establishment of WTEs for band 6 nurses will still be 1.5.