Blocks are the containers that give a pane its layout. They snap to the pane grid, can span any number of cells, and never overlap. All content lives inside blocks.
Drag a block to move it between cells; drag its edges to extend or shrink the cells it spans. Need positions the grid does not offer? Increase the pane's grid resolution - see Working With Panes.
To delete a block, select it and press .
This is the heart of PaneFlow transitions. Select a block and press CTRLC ("Copy Block Reference"), then right-click an empty cell on another pane and choose "Paste Block Reference". The same block now lives on both panes - same content, independent grid placement - and PaneFlow animates between the two placements during navigation.
Transitions & Morphing covers what you can do with this in depth.
With a block selected, the settings sidebar shows:
Right-click a block to "Copy Block Styles" (CTRLC) and paste them onto another block (CTRLV). "Set As Default Block Styles" makes the current look the starting point for every new block in this project.