Hi Michelle,
In past versions of WFM, shifts with Task Sequences & shifts with Activity Sets could not be built together in the same build. This can now be done but for WFM Builder to know how to correctly handle these shift items the setting has been introduced.
Leaving this box unchecked will mean that Builder will be operating in Task Sequence mode for the agents with that contract and may cause unexpected results if the shifts assigend to the contract do not have Task Sequences assigned to it... and vice versa if you have the box checked and the shifts have Task Sequences assigned.
If you do not use Task Sequences at all in your business, just tick the box on all your contracts and you shouldn't notice a difference.
Have fun :)
am