That symbology means this approach cannot be considered when deciding if this airport is a suitable alternate. This is probably due to the lack of weather reporting or the navaid might not have the required monitoring. This means the availability of the approach cannot be guaranteed and so you can not rely on it as an alternate.
To file TTA as an alternate you must comply with the the non-standard alternate minimums and you must be legal and equipped to fly either the RNAV (GPS) RWY 03 or RNAV (GPS) RWY 21 procedures. If the ILS Y RWY 03 is available when you get there, you may fly that procedure instead of the RNAV procedures, you just cannot file it as an alternate on the basis of the ILS.
The wording in the Terminal Procedures legend (PDF) is
Alternate minimums are Not Authorized due to unmonitored facility or absence of weather reporting service