The Part Library is organized in tabs. Please note that these tabs will always appear in English no matter which language is chosen for the software. This is because the titles reflect the names of the folders in which the parts are located. If you rename the subfolders in the BlueBrick parts folder, this change will be reflected in the tab titles (after reloading the Part library from File > Reload Part Library or relaunching BlueBrick). It is safe to do it because BlueBrick does not use the folder names to reference the parts in your layout.
Some parts of the official library may also change of tab from one version of BlueBrick to another, due to reorganization done by the BlueBrick Part Manager (who is usually a member of the community of LEGO® Fans), which also doesn't affect the layouts created by the BlueBrick users.
The "Custom" tab provides some unofficial parts, or the parts you may create yourself by grouping some existing parts (see How to save a group in the Library section). After saving your group in the "Custom" tab, you are, of course, free to move it to another tab, or even to a new tab you may create yourself. Some Parts in the Custom folder are also here for compatibility reasons with Track Designer.
Inside a tab, the parts are sorted first with there "sorting key" defined in the XML file of the part, and then with their name (see the Sorting Key section for more details). The "sorting key" of the official parts are chosen by the Part Manager who usually uses categories to group the parts in a logical manner, like for example: Creator, City, Modular, etc...
The order in which the Tab are listed in the Library can also be customized in order to let you display the tabs you use the most at the beginning. To customize the tab order, please read the Part Library Preferences section.
BlueBrick is welcoming in its Official Library, all the official LEGO® Set plus some individual parts that are useful for Layout planning like baseplates, road and train track. The Official Library can also host some widely spread custom modification among LEGO® Fans like for example half rail tracks. But the library does not intend to host all the individual LEGO® parts existing as the LDraw Library does. However the Official BlueBrick Library follows the LDraw naming convention to avoid any name clash.
Since the introduction of the Grouping feature in BlueBrick, Official Sets are often implemented as a Group which can be ungrouped once added on the layout. So if you just need a sub element of a Set on your layout, first search for the whole Set in the Library, then add it to your layout and ungroup it. Finally you can cherry pick the sub element you want and delete the rest.
The arrangement of the sub elements of an Official Set tries to follow the arrangement depicted on the picture of the box, but if you don't like it, you can ungroup the set and rearrange the elements (for example you can move the car of the set, far from the buildings).
Official Sets that have hinged bricks (like for example a house that can be open in two) are usually implemented with the Flex feature, so you can also adjust the opening position if you don't like the default one (as it can be seen with the Town Plan Set example beside).
The appearance of each Part Library Tab can be tuned by right-clicking in it. It will bring a contextual Menu in which you can choose some options: