The fillet weld must have fusion to the root, but not necessarily beyond the root if you are working to AWS structural codes. However, if one is investigating skewed T-joints, it is likely one will not have fusion to the root. In that case, one needs to verify the incomplete fusion doesn't exceed the Z-loss listed by the applicable structural welding code.
As for the depth of fusion in the region of the fusion faces, the depth of fusion is not a dimension that is specified, only that the weld be fused to the base metal. Excessive depth of fusion can be detrimental in some cases due to excessive dilution.
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