To customise AutoCAD P&ID Line Numbers you will set it up in the project setup. This is accessed by right clicking on the name of the project in the project browser and clicking on properties.
Expand the plus button next to the Engineering Items underneath the P&ID Class Definitions.
To modify the Tag format (found at the bottom of the dialog box ) you need to have the Pipe Line Segments selected (found in the left panel). At the top of the dialog box you will see that you are editing the Pipe Line Settings.
The default Pipeline Tag has the following format. 
The tag segments represents the folllowing.
- Size
- Spec
- Pipe Line Group Service
- Pipe Line Group Number
You can create a new tag format by clicking on New. The Tag Format Setup Dialog box opens.
Format Name – The identifying name of the tag
Number of Subparts – How many subparts you have for the tag
You have 4 different icons for the different properties you can access for the Tag.
- Displays the Select Class Property dialog box
2.Displays the Select Drawing Property dialog box
3.Displays the Select Project Property dialog box
- Displays the Define Expressions dialog box
Once you have populated your number of sub-categories with the correct information you need to set that tag to be used when you draw your pipe lines.
Scroll to the bottom of the properties section. Next to TagFormatName click on the drop down arrow and select the tag that you just created.
