This register has one bit for every layer that indicates whether a layer is enabled (1) or disabled (0). Bit[0] is for the master input layer, which associates with the s_axis_video AXI4-Stream input. This is the bottom-most layer, and all other layers are blended on top. If this layer is disabled and no other layers are blended on top, the background color as specified by the previous mentioned registers are shown. If this layer is enabled, video data should be sent to this layer, otherwise the Video Mixer stalls.
Bit[1] is for overlay layer 1, which associates with either s_axis_video1 AXI4-Stream input or m_axi_mm_video1 memory mapped AXI4 input, depending on the Interface Type field selected in the Vivado IDE. Bit[2] is for overlay layer 2 and so on, until Bit[16] which enables/disables layer 16.
Bit[23] is for the logo layer. The logo layer is the top-most layer and is blended on top of all other layers.