There are many settings you have control over which are used by the Build Engine. You can set your defaults in the Building tab of your > page. Plus, you can override the default settings for any individual Build you perform.
There are three options for this setting (Exact, Similar, Ignore). They determine how close the color of the source and target parts need to be to consider them a match.
Exact - only parts with exactly the same color are matched, e.g. the first three parts below are considered the same:
Similar - parts with similar colors are matched. If you are on the Pro Plan you can customise what colors are considered similar, e.g. the first three parts below are considered the same:
Ignore - match parts regardless of their color, e.g. all the parts below are considered the same:
The Rebrickable admin team maintain the relationships between parts such as Prints and Mold variations. These relationships are used by the Build Engine to allow for very powerful matching techniques.
You can ignore any prints on parts so that they match with their unprinted versions, or even with the same part having a different print, e.g. these 4 printed tiles will be considered the same:
You can ignore the mold variations that lead to multiple of the same part. For example the three parts below are actually different (look closely at the claw) and were sold at varying times. This option will consider them the same.
Some parts are almost the same, but not quite. In the example below, the first part has stud holes in the edges of the wings whereas the second part does not. This might impact where the parts can be used, but it depends on what you are building. This option will consider them the same parts.
Some of the parts found in typical LEGO Sets or MOCs are not really required to build them, such as Minifigures, Books, and Stickers. There are a couple of options to exclude these parts from the Build Engine calculations completely.
In addition to the Build Engine settings above, there are a number of settings that control what Build related notifications you receive.
Build notifications are sent when new MOCs are published that meet your minimum criteria. For example, you can elect to receive a notification via email when a new Technic MOC is added and you have 95% of the parts required to build it according to your default build settings detailed above.