Attribute Holder is a modifier in 3dsmax, which helps us to add a custom attribute to an object in 3dsmax. For example, if a Bend modifier is applied over a Box object in 3dsmax, you could either control the Bend parameters or the Box's parameters by switching between the Bend modifier and the Box object.
In the above image you can see Bend Modifier applied over a Box. As said before, you need to select the respective object to edit its parameters. Instead of switching between the Bend modifier and Box object, we can use a single modifier that holds the attributes of Bend as well as the Box in one place. In this tutorial let us see about the Attribute holder modifier.

First place a Box object with the following dimensions (Length and width = 20 units, Height = 50 units).

Next apply a Bend modifier. Now we are about to control the Bend angle and Box width from one place.

Now apply the Attribute Holder to the stack. (Refer image below).

Box, Bend modifier and Attribute Holder

Attribute Holder modifier shows an empty panel, in which you will put your custom attributes. Now from the menu select Animation -> Parameter Editor , which will open the Parameter Editor Dialog.(Refer image)

In the Parameter Editor Dialog use the following settings and click - Add

Parameter type = Float
UI type = Spinner
Name = Bend angle

Now you must see an attribute named Bend Angle added to the Attribute Holder panel. (see image below)

In the same way add another attribute named Box Width. Now from menu select Animation -> Wire parameters -> Wire parameters and connect these attributes.

Modified object -> Attribute holder -> Bend Angle with Modified Object -> Bend -> Angle.

Now follow the same step to create another attribute for Box length and wire it.

for wiring parameters read the last tutorial Animate car wheels through wire parameters.

More 3d models
Realistic electron microscope material in 3dsmax
Volume Select Modifier in 3dsmax – 3d tutorial
Shell Modifier in 3dsmax
Reactor – Toy Car in 3dsmax