This screen is used to create/view/edit the Units of Measure lookups. These are used to set the units available for the Selling, Dispensing and Packaging Units on the Product Information screens.

The fields are as follows:

Name - the name of the Unit of Measure (eg Unit, Grams, Pounds, Box, Syringe, Pack of 10)
Printed Name - this is used to provide a short abbreviation that can be used when displaying quantities on invoices, credits, counter-sales and statements. See also below.
Unit Code - this is used by the ESCI facility (see below).  If you are not using ESCI, then you can happily leave this set at its default of 'each'. 
default - check this box to make this the default Unit of Measure
Active - uncheck the box to deactive the Unit of Measure

The Printed Name field can hold a maximum of 5 characters.  However the standard invoices etc in the release package allow space for 3 m characters, ie 'mmm'. Note that in the majority of cases you can leave this field empty and only set it where needed. Thus although
    Hills Feline D/D Venison Can 5.5oz   10
makes sense, you probably do want
    Atropine Injection                          0.25 ml

It is probably worth pointing out that the standard convention is that unit abbreviations should be singular (ie ml rather than mls).

The Unit Code is used by the ESCI facility to map UN/CEFACT Unit Codes* to the practice equivalents. The list contents are defined within the archetype lookup.uom and can only be added to by modifying the archetype. Note that OpenVPMS only provides a subset of the available codes, so if a required code is missing, it can be added but must be present in the list provided by http://docs.oasis-open.org/ubl/cs-UBL-2.0/cl/gc/cefact/UnitOfMeasureCode...

* UN/CEFACT Recommendation N°. 20 - Codes for Units of Measure Used in International Trade - http://www.unece.org/fileadmin/DAM/cefact/recommendations/rec20/rec20.zip