This Dravidian alphabet is exclusive to Malayalam among Indic scripts. It stands for both double and single of the sound /ta/ as in റ്റുമാറ്റൊ (single) and പറ്റി (double). For native Malayalam words, this sound never exists outside a conjunct or a double. For words adopted from English, the same റ്റ could represent single /ta/ sound as well. Right now റ്റ is being generated in Unicode as RRA + VIRAMA + RRA. That is, റ്റ is considered the double of റ which sounds /rra/ and obviously different from /ta/.
It is not wrong to write the word ‘സിറ്റി’ as ‘സിററി’. These two words have, different Unicode representations. Instead, Unicode equates ‘സിറ്റി’ to ‘സിറ്റി’ which unfortunately is wrong. That essentially means, Unicode's position of /ta/ as the double of /rra/ is not achieved.
If given a codepoint, റ്റ can form conjuncts with consonants like ന, സ etc to get ന്റ, സ്റ്റ etc. with റ്റ being in the C2 position.