Karnatakabhashabhushana 8.14

For its singular, tu and tū [are substituted] optionally according to others.

For the third-person singular prescribed in the neuter gender, tu and tū are optionally substituted according to the opinion of others.

puṭṭitu,

puṭṭittu,

sogayisitu,

sogayisittu,

aḷkitu,

[and] aḷkittu. By fixed option, in some cases it is something else altogether: kēṭṭattu,

sattattu,

ādattu,

[and] pōdattu.