Bluebrick has a convenient feature to group several parts together (read the Grouping Parts section for more details). This is indeed convenient to move around a complex arrangement of parts on your map, but you may want to save this group in the Part Library for a later use in another map. Fortunately this possible: you first need to select a group or several parts on your map and chose the File > Save Selection in Library... menu (if none or only one part is selected the menu will be disabled); this will display this window:
Then you must give a unique name to your group in the text box 1. This unique name will be used as the part id and also as the name of the file created to save the group on the disk. This is why you cannot used special characters like the question mark "?". The text box will stay red as long as the name is not valid. BlueBrick suggest you ".SET" as an extension, according to its own convention, but you can choose another extension.
The XML file containing the description of your group will be saved in the "parts\Custom" folder of the BlueBrick installation folder. If this "Custom" folder doesn't exist, it will be created. However, you can move this file to another folder with the file browser of your operating system, and then reload the Part Library.
You can also specify a sorting key for your group in the text box 2, such as you can control where it is listed in the Part Library (read the Sorting Key section for more details).
The checkbox 3 let you choose if you want your group to be un-groupable or not. Most of groups should be un-groupable for a maximum of flexibility for the user, however if you're creating a set in two parts linked by a hinged brick, you may not allow the user to separate the set.
The Author text box 4 will display your name if you have set it in the Preferences otherwise just "Unknown Author" as default. The goal is just to give you credit in case you decide to share your group on the internet. Type anything you want or leave it blank if you're not interested by fame and glory.
Then you can type a description for your group in different languages: First you should choose the language code from the combo box 5 and then type the description in that language in the text box 6. You can of course enter a description in several languages.
The Image URL text box 7 is useful if you're creating an official set whose image can be found on internet, then please copy/paste the link here such as BlueBrick can use it when exporting the Part list. The Part URL 8 is not used yet in that version of BlueBrick.
Once you have finished to fill the form and clicked Ok, the part will be saved on disk and the Custom Tab will be selected in the Part Library to show your new part. The new part can be used right away by dragging it on your map. Beware that if you plan to share your map, you will also need to share your new parts with your friends, otherwise they will just see a red cross in place of your parts when opening the BBM file you sent them.