You can find the Testpoint Assign Tool under Plugin >> Assign Testpoint Attribute.
This tool helps to find and mark testpoints in a layout which could be defined via multiple non standardized attributes. Marking them with this tool then helps to standardize this and prepare the layout for following tools (for example the DFT Preperation).
On the top left you can switch between using the current selection or a set rule to find the testpoints and assign the test_point value.
When choosing rules you can add another rule by checking the checkbox of the lowest row in the current listview and then fill out which attribute should be tested and what value needs to be set in said attribute.
All attributes are listed in a simple drop down menu. The Filter Value then has to represent the value of the attribute. Wildcards are allowed when searching for attribute values. The image shows for example all ways to search for "tp" in the "geometry" attribute:

• The first rows searches for all geometry attributes that have exacly the value "tp".
• The second row seraches for all geometry attributes that end with "tp"
• The third row searches for all geometry attributes that start with "tp"
• The fourth row searches for all geometry attributes that have "tp" anywhere in their value

Also it is possible to search for multiple values of the same attribute as shown in the last row. Simply seperate the values with a semicolon.
Boolean attributes do not need any value since their existance already means they are set.
To remove a rule simply select the whole row by clicking on the left most column of the row you want to delete (left of Use Rule) and press the "delete" key on your keyboard.

Checking a rule will select all found possible testpoints of the job (the checkboxes Assign Top and Assign Bottom change the selection depending on the check state of them)

On the top right side there is an information panel showing:

•  The currently already set testpoints of the loaded job for the top layer and the bottom layer
•  A button to open this help page
•  A button to select all currently set testpoints of the loaded job

Export Rules exports the currently set rules to a xml file on selected location on the machine or in a cloud
Import Rules imports rules from a file either from the machine or from a cloud
Remove all Testpoint Attributes removes the "test_point" attribute from all pads of the job
Assign Testpoint Attribute adds the "test_point" attribute to either the currently selected pads or the pads found by the selected rule (handled by the radiobuttons "using Selction" and "using Rule")
Assign Top is a checkbox that when unchecked the "test_point" attribute will not be set for the pads on the top layer
Assign Bottom is a checkbox that when unchecked the "test_point" attribute will not be set for the pads on the bottom layer

To find possible testpoints the attribute histogram is a good way to start.