Max. Units, the maximum units value specifies how much of this resource is available for this project — for example, part-time or multiples.
Saying this resource is available 100% means he is a full time resource, less than that means he is a part time resource.
For example, if you have a resource who is available 3 days a week then the Max Units for him/her is 60%. If he/she is available for the whole week then Max Units is 100%
You can enter maximum units as a percentage (50%, 100%, 300%), or as a decimal (0.5, 1, 3).