special numeric missing value

Generally, we judge the numeric missing value with "varname = .".
However, it is not good SAS habit because it may miss special numeric missing value.

From online doc, we can get the definition of Special numeric missing value:
is a type of numeric missing value that enables you to represent different categories of missing data by using the letters A-Z or an underscore.

That means there may be at most 28 numeric missing values as follows:
._
.
.A-.Z

To avoid unexpected missing, we should use MISSING and NMISS for convenience.

For more explanation, we can read SUGI31 paper: MISSING! - Understanding and Making the Most of Missing Data