The TPP has a section containing alternate minimums for approaches. It starts off with this paragraph.
Standard alternate minimums for non-precision approaches and approaches with vertical guidance [NDB, VOR, LOC, TACAN, LDA, SDF, VOR/DME, ASR, RNAV (GPS) or RNAV (RNP)] are 800-2. Standard alternate minimums for precision approaches (ILS, PAR, or GLS) are 600-2. Airports within this geographical area that require alternate minimums other than standard or alternate minimums with restrictions are listed below. NA – means alternate minimums are not authorized due to unmonitored facility, absence of weather reporting service, or lack of adequate navigation coverage. Civil pilots see FAR 91. IFR Alternate Minimums: Ceiling and Visibility Minimums not applicable to USA/USN/USAF. Pilots must review the IFR Alternate Minimums Notes for alternate airfield suitability.
The first part is straightforward. In order to use an approach as an alternate when filing an IFR flight plan, the visibility has to be 2 statute miles and the ceiling has to be 800′ AGL if the approach is non-precision. It has to be 2 statute miles and the ceiling has to be 600′ AGL for precision approaches. If the minimums are different for an approach then the non-standard minimums are listed. They can be things like NA when control tower closed. or NA when local weather not available. as well as increased visibility and ceiling. The minimums are specific to each approach.
What confused me is the NA notation. When I first saw it on the chart, I assumed that the approach was not available as an alternate. But when I read the note above, NA – means alternate minimums are not authorized due to unmonitored facility, absence of weather reporting service, or lack of adequate navigation coverage. I was a bit confused. Does it mean that alternate minimums are not available and you can use standard minimums? Or does it mean that no alternate minimums apply to this approach, therefore it can’t be used as an alternate.
It turns out that what it means is that the approach is not available to be used as an alternate when filing an IFR flight plan. If there is another approach that is available for use as an alternate, and the pilot needs to fly to the airport, the approach can be flown if the approach minimums are met at the time of arrival. Andy Munnis explains it well (beginning at the 51 minute mark):