Clicking the Fill button in the Parts panel displays the following dialog in the lower region of the panel :

This dialog lets you fill the selected Part with materials and initial conditions.
The only option available for unstructured parts is to fill the whole part. When an unstructured parts only needs to be filled partially, the Group option can be used to create a selected group of elements which then can be filled with materials or initial conditions.
Fill by Index Space
- Block
 Fill a block in index space.
- I/J/K Plane
 Fill a plane in index space.
- Node
 Fill a node.
- Unused
 Fill a block in index space with unused elements.
- Composite
 Fill a composite shell.
Fill Multiple Parts
- Multi-Fill
 Fill multiple Parts.
Additional Fill Options
- Datafile
 Read or write a datafile for General Remapping.
- Part Fill
 Fill an Euler Part using Lagrange or Fill Parts.
- Block Remap
 Read or write a datafile for Fast Block Remapping.