If the engunits of the PI Point happen to match a UoM abbreviation of a UoM in AF, then some conversions between UoMs are done automatically. But otherwise there is little match between the two.
But could you elaborate a bit on your question? What "use" do you need? And what do you mean by "use from PI-AF"?
I would recommend you to check white paper by @Anna Perry related to AF UOM and PI Eng units. In PI you can enter any text in engunits field but in AF it is predefined set with extensibility. During Tag build and AF build, you can match with AF set match as per instrument engunits. AF allows you to convert one unit to other in pinpoint settings where you can add source eng unit in attribute and conversion in Default UOM.
Are you trying to get the UOM attribute of the PI Tag from PI Data Archive to PI AF? PI Analysis does not provide an expression (like TagNum, TagType, TagName etc.) to retrieve the UOM property of the PI Tag. This is reasonable as PI Data Archive can accept any string in its UOM attribute, but when it comes to PI AF, the UOM must comply to the UOM Database. Anything that is not in the UOM Database cannot be used unless you have defined them explicitly. For example, in PI Data Archive, you can use "sec" or "s" for "seconds" but in PI AF you have use "s" because it is UOM abbreviation that is defined in UOM database for seconds is "s". You have to set the UOM's when you configure the attributes for the AF Element or Element Template, which must match the units in the source system that PI Data Archive connects to.
Once you have defined the UOM's in AF, you can convert them from one unit to another, given the units that you are trying to convert to and from should belong to same UOM Class. For example, seconds to hour can be done since they both belong to UOM Class "Time". Once it is setup in AF, the client applications can make use of this explicit conversion. You can also make request for specifics units from your code, without worrying about doing the conversion yourself. As Roger has pointed out, you need to be clear on "what's the use".
Regards, Jaison Rodrigues