Hi,
Stefi is referring to is a 'Multi-Value' Attribute's 'Ext.Properties'. (for MX_PENDINGVALUE).
In the approval task screenshot (above) pending value isn't listed.
Checked in my sandbox and find that the 'Reason' field (highlighted in red rectangular box) is coming even if no attribute is listed (as mentioned by Samrat).
Guess it is safe to assume that this field is built internally and presented for an approval task in UI. Will be interesting to know the solution for making the field as Mandatory.
BR,
Ganesh.S