A good explanation in the link provided by CheterCopperpot above.
There are multiple transfer slabs in The Well complex - some of which are still visible in photos being taken now. In the photo immediately above, the two buildings to the right (east) of the one where the pour is taking place today, each one has a visible transfer slab. The slabs are where the set back are from the commercial floors below the slab and the residential floors, just starting to be built, above. The slabs look to be around five to six feet thick. The structural load bearing columns for the residential floors do not line up with the structural columns for the commercial floors below them. The transfer slab is built to handle moving the load from the upper set of columns to the lower ones.
Other transfer slabs are located in the below grade areas of the complex, above the service / truck access driveway makes its way through the basement. Given the width of the driveway, transfer slabs are used to provide a clear span across the drive ramp while supporting the load bearing columns in the commercial areas above.
A side note on the transfer slabs for the six residential buildings. RioaCan and Allied, the developers of the commercial components of The Well (the office tower, the below grade areas, and the commercial areas in the podium levels) sold the residential development rights in the complex to Tridel and Woodbourne. Payment for the the residential development rights was tied to completion of the transfer slabs for the residential towers - payment coming due on a building by building basis once the transfer slab between the commercial and residential components was completed.
Here is an image of the transfer slabs on two of the Wellington Street residential towers - the light grey bands of concrete between the bricked residential portions of the building and the commercial components below. As can be seen, the structural columns above the slab do not line up vertically with the columns below the slab.
