It's called the Estimated Sign.
"The estimated sign is a mark required to be appended to the nominal mass or volume printed on prepackaged goods for sale within the European Union. It certifies that the actual contents of the package comply with specified criteria for estimation:
- the average quantity of product in a batch of prepackages shall not be less than the nominal quantity stated on the label;
- the proportion of prepackages having a negative error greater than
the tolerable negative error shall be sufficiently small for batches of
prepackages to satisfy the requirements of the official reference test
as specified in legislation;
- no prepackage having a negative error greater than twice the tolerable negative error may bear the estimated sign.
The tolerable negative error is related to the nominal quantity and varies between 9% on prepackages nominally 50 g or mL or less, to 1.5% on packages nominally 1 kg or L or more.
The estimated sign looks like a lower-case "e" and its shape is precisely defined by an EU directive. This sign has been added to the Unicode list of characters, just as the symbol for the Euro has been, and can be considered a new basic character in a typeface. It is character U+212E."
The estimated sign is placed either before or after the net weight on European packaging. I don't believe there is a consistent location for it, since it's placed on either side for no apparent reason. The European net weight in grams, milliliters or liters only. Ounces are not generally stated, but are sometimes included alongside the net grams.