KB: Assign footprint information to mechanical layer in a PCB library

Solution Details

As there is no standardization set forth on mechanical layer assignment, different footprint libraries (*.pcblib) may contain supplementary information on different mechanical layer indices.  With no strong preference, the best bet is to follow that of IPC Footprint Wizard (Tools » IPC Compliant Footprint Wizard) as your starting base.
 

As a reference, here is a non-exhausitve and conflicting list of layer usages from various sources.

From Altium Designer's IPC Footprint Wizard:
Mechanical 1 Text Notes
Mechanical 13 Assembly Outline (Top)
Mechanical 13 Component Body / Assembly
Mechanical 15 Placement Courtyard (Top)
Mechanical 12 Assembly Outline (Bottom), paired with Mechanical 13 
Mechanical 14 Placement Courtyard (Bottom), paired with Mechanical 15

Most Altium examples:
Mechanical 16 TitleBlock.

From Altium templates designs:
Mechanical 1 Board Outline (Note: board outline should not include text)
Mechanical 3 Mechanical Part details
Mechanical 4 Dimensions and other call outs.

From Vault templates:
Mechanical 1 PCB Boundary
Mechanical 2 Gerber ID
Mechanical 3 Printout ID
Mechanical 6 Fab Notes (Comments) 
Mechanical 7 3D Component Body for free models
Mechanical 15 Placement Courtyard

From older examples:
Mechanical 1 PCB Board Outline, (Also Component Bodies, board cutouts, legacy slot details) 
Mechanical 2 Gerber ID 
Mechanical 3 Printout ID 
Mechanical 4 Dimensions (sometimes Printout ID) 
Mechanical 5 Mechanical data 
Mechanical 6 Fab Notes (Comments) 
Mechanical 7 Assembly Top 
Mechanical 8 Assembly Bottom 
Mechanical 11 Design Info 
Mechanical 16 Title Sheet (typically linked to sheet from 'Design » Document » Options » Link to sheet')

Ultimately, it is therefore highly recommended to pre-assign Layer Types to each of mechanical layers in your library, so that in an event you come across a library, from which you want to copy a footprint, where mechanical layer indices are assigned differently, Altium would still be able to move whatever primitives on a particular mechanical layer index from the source library to the corresponding mechanical layer index of the same Layer Type in the destination library.

1. In respective libraries (both source and destination *.pcblib's),  open View Configuration panel (View » Panels » View Configuration)
2. On Layers & Colors tab, under Layers section, double-click on the mechanical layer of interest to open Edit Layer dialog.
3. Select a Layer Type from the pull down
4. Repeat step 2-3 for all mechanical layers
5. Copy footprint(s) in PCB Library panel (View » Panels » PCB Library) from the source library with Ctrl+C and paste in the panel of the destination library with Ctrl+V

Please read further below on Layer Type:
https://www.altium.com/documentation/altium-designer/working-with-mechanical-layers#!the-advantage-of-assigning-a-layer-type

Assembly layer type is somewhat of legacy before the time of 3D Body and Courtyard, so most probably the former is not needed if the latter exists.  Additional reference on how mechanical layers determine the component selection bounding box:
https://www.altium.com/documentation/altium-designer/placing-components-pcb#!component-selection-bounding-box

Mechanical Layers can be imported/exported to/from *.pcbdoc and *.pcblib documents in menu Tools » Import/Export Mechanical Layers:
https://www.altium.com/documentation/altium-designer/working-with-mechanical-layers#!exporting-and-importing-mechanical-layers
Please note here this differs in operation from simply copying/pasting a footprint or any primitive from one document to another in a sense that the entire definition inclusive of Layer Type is overwritten by source document to destination document.