From XFOIL’s manual (which is what XFLR5 uses under the hood for airfoil analysis):
Airfoil file formats
--------------------
LOAD recognizes four airfoil file formats: Plain, Labeled, ISES, MSES.
All data lines are significant with the exception of lines beginning
with "#", which are ignored.
Plain coordinate file
.....................
This contains only the X,Y coordinates, which run from the
trailing edge, round the leading edge, back to the trailing edge
in either direction:
X(1) Y(1)
X(2) Y(2)
. .
. .
X(N) Y(N)
Labeled coordinate file
.......................
This is the same as the plain file, except that it also has an
airfoil name string on the first line:
NACA 0012
X(1) Y(1)
X(2) Y(2)
. .
This is deemed the most convenient format to use.
The presence of the name string is automatically recognized if
it does not begin with a Fortran-readable pair of numbers. Hence,
"00 12 NACA Airfoil" cannot be used as a name, since the "00 12"
will be interpreted as the first pair of coordinates. "0012 NACA"
is OK, however.
Some Fortran implementations will also choke on airfoil names
that begin with T or F. These will be interpreted as logical
variables, defeating the name-detection logic. Beginning the
name with _T or _F is a workable solution to this "feature".