what a. means in a observation?

[Alejandro Torres]

Hello, I merged two databases and after that, some observations (cells) have a string "a."
What does it mean?
Thank you

[Alejandro Torres]

Dear Daniel and Richard, effectively is .a, thank you very much!

[Richard Williams]

Are you sure it is not .a? .a is a missing value.

[Daniel Devine]

It is a missing value. See this link: https://www.stata.com/help10.cgi?missing+values