Which columns can be encrypted?

I'm going to have some requirement to encrypt some columns in 1ID. I'm not sure which ones yet. I can see that in Designer in One Identity Schema, I can set a column to be encrypted, then I run the encryption tool again and it will be encrypted in the database. I've noticed that some columns do not allow encryption. Why is this? Is there any pattern to which columns can be encrypted and which cannot? If the encrypted checkbox is greyed out for a column, is there any way to encrypt that column. Using version 8.