What is the "reserved meaning" of an ampersand (&) in column 9?

[dtdoering]

From the spec (emphasis mine):

In addition, the following characters have reserved meanings in column 9 and must be escaped when used in other contexts:

; semicolon (%3B)
= equals (%3D)
& ampersand (%26)
, comma (%2C)

and

Column 9: "attributes"
A list of feature attributes in the format tag=value. Multiple tag=value pairs are separated by semicolons. URL escaping rules are used for tags or values containing the following characters: ",=;". ...

I understand that semicolons (;) and equals signs (=) are for separating tag-value pairs, and that commas (,) are used when multiple values are assigned to a single tag, but what are ampersands (&) for? I don't see it mentioned anywhere in the spec, nor the pathological cases, and my best attempts to Google it have come up empty.

Thanks for any insight!