This is a simple way for a user to specify that they don't want a particular entry "Suggested" to them in AutoFill, for example in the Dropdown suggestions menu in the browser.
To do this Strongbox uses a CustomData key/value pair like this:
"KPEX_DoNotSuggestForAutoFill" = "True"
Where the value is the standard KeePass boolean string "True". Here is what the KeePass XML entry looks like:
<Entry>
<UUID>3m+H+PlcSASzTVYaDNFyVA==</UUID>
<String>
<Key>Title</Key>
<Value>My Entry</Value>
</String>
<String>
<Key>UserName</Key>
<Value>user</Value>
</String>
<String>
<Key>Password</Key>
<Value Protected="True" />
</String>
<Times>
<LastModificationTime>p4JE3A4AAAA=</LastModificationTime>
<CreationTime>nYJE3A4AAAA=</CreationTime>
<LastAccessTime>p4JE3A4AAAA=</LastAccessTime>
<Expires>False</Expires>
<UsageCount>2</UsageCount>
<LocationChanged>nYJE3A4AAAA=</LocationChanged>
</Times>
<CustomData>
<Item>
<Key>KPEX_DoNotSuggestForAutoFill</Key>
<Value>True</Value>
<LastModificationTime>p4JE3A4AAAA=</LastModificationTime>
</Item>
</CustomData>
</Entry>