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Standards
Some laptop components (optical drives, hard drives, memory and internal expansion cards) are relatively standardized, and it is possible to upgrade or replace them in many laptops as long as the new part is of the same type. Depending on the manufacturer and model, a laptop may range from having several standard, easily customizable and upgradeable parts to a proprietary design that cannot be reconfigured at all. The replacability/upgradability of the hardware can be announced as positive by the laptop maker; a few brands sell "barebones" laptops which can be outfitted by the purchaser.
In general, components other than the four categories listed above are not intended to be replaceable; a few, such as processors, follow their own standards but are difficult to replace because of other factors (for example, in the case of processors cooling and access limitations can make upgrades very difficult or impossible.)
In particular, motherboards are almost always make and model-specific: locations of ports, and design and placement of internal components are not standard. Those parts are neither interchangeable with parts from other manufacturers (replaceable) nor upgradeable. If broken or damaged, they must be substituted with an exact replacement part. Those users uneducated in the relevant fields are those the most affected by incompatibilities, especially if they attempt to connect their laptops with incompatible hardware or power adapters.