This is a string, either 'OPEN' or 'CLOSED'. (Only the first letter is actually used.) Use 'OPEN' if the tube is an open framework, and 'CLOSED' if it is an opaque cylinder. If (as in some telescopes) the part near the declination axis is closed, but the front end of the tube is open, use 'OPEN'. This information is needed to determine when the Moon can shine directly on the objective.
If the telescope is a refractor used without a dewcap, use 'OPEN'. A refractor used with a dewcap should be marked 'CLOSED'; the dewcap information will be used in the TUBEDIAM and TUBELEN columns (see below).