Project entry date is what is known as a "native field" -- it came with your Workfront environment and was provided as a way to store the date that the project was created. All customers have this field.
Project submission date is what's known as a "custom field" -- it was created by your system admin as an additional data element to be stored for your workflows. Likely many customers will either not have this field, or it may work differently or be named something different.
Additionally, custom fields must be placed on your objects via a custom form, whereas native fields simply exist without needing to be placed.
