Drawings, Templates, and Layers
Templates
A drawing template (.dwt) is the fastest way to guarantee that every drawing in a project starts with the same layers, text and dimension styles, layouts, title block, units (INSUNITS, LUNITS, LUPREC) and named page setups. QNEW opens the file named in Options > Files > Template Settings > Default Template File Name for QNEW, so a firm can point every new drawing at its own template instead of acad.dwt. MEASUREMENT (0 imperial, 1 metric) belongs in the template because it drives default hatch patterns and linetype files.
Layers
Layer 0 cannot be renamed or deleted, and objects on layer 0 inside a block take on the colour and linetype of the layer the block is inserted on - that is the mechanism behind the geometry-on-layer-0 block practice. Defpoints is created automatically by dimensions and never plots. VP Freeze hides a layer in one viewport only; Freeze and Off are global, and frozen layers are not regenerated at all. Locked layers still permit object snaps and plotting but not editing. Layer states (.las) save on/off, freeze/thaw, colour, linetype and plot status, and are restored through LAYERSTATE or the Layer Properties Manager.
Worked example
A floor plan must show grid lines in the overall viewport but not in an enlarged detail. Create the detail viewport, double-click inside it to make it current, then VP Freeze the grid layer. Do not turn the layer Off and do not Freeze it globally: both would remove the grids from every viewport, including the overall plan.
Traps
Attempting to rename or delete layer 0; expecting Defpoints content to plot; freezing the current layer (AutoCAD refuses and reports an error); assuming a template page setup exists in a drawing that was never started from that template.