When you set up a chart with a Daily periodicity, Investor/RT downloads Daily bars from the exchange and receives back 5 values; Open, High, Low, Close and Volume for the session. These values are calculated by the exchange using what they deem to be the official open and closing price and the Volume that occured between the Open and Close. The Official Closing price is generally not posted until a few hours after the close so the Daily Bar that is downloaded can have a different Closing price a few hours after the session close time. This data covers an entire session so regardless of the session you have set for your instrument or chart in Investor/RT, the values will be the same.
If you want to be more specific with your analysis, use the periodicity called 1 Day* or use a periodicity of the number of minutes in your session. When using this, Investor/RT builds the a Daily bar for you using 1 Minute data. This allows you to view Daily bars for a particular session (i.e. the Regular Trading Hours session only). Using this method, your closing price will be the Last Price observed when the session closes and there will be no adjustment after that.