A full-fill cavity wall has insulation occupying the entire width of the cavity, rather than leaving a clear air gap between the insulation and the outer leaf. It allows a better U-value for a given cavity width, but it depends on correct materials, workmanship and detailing to avoid moisture transfer.
In partial-fill construction, insulation boards are held against the inner leaf and a clear residual cavity remains next to the outer leaf. In full-fill construction the whole cavity is insulated, usually with mineral wool batts, insulated bead or a pumped product, which gives more insulation for the same overall wall thickness.
The trade-off is moisture management. Full-fill systems must be suitable for the exposure conditions of the site and installed without gaps or mortar bridging, so certification of the system and careful supervision on site are important.