Often x and y errors are known as a formula.
Examples:

Poisson statistical error in a count, where the uncertainty in the count
is given by the square root of the count

measurement errors using a ruler where the uncertainty is constant
and taken as half the finest division on the ruler

measurement device specifications, which often list likely
calibration errors as a formula (see below)

Alternatively, error estimates may be individualized and not expressible
as a simple formula.
Examples:

judgments about when a condition applies (problem of definition errors) like:
judging the range of lens locations that results in a focused image or
judging if the star is exactly in the crosshairs now

errors found by the deviations observed on repeated re-measurement.

In the former case (where the error can be reduced to a simple equation),
WAPP^{+} provides a way to type in the formula rather than
calculating the result for each datapoint. Much like a spreadsheet you can
type in your error formula using the usual fortran syntax:

Example: DMM Specifications

What follows is a simplified version of the "resolution and accuracy"
of the DM-441B Digital Multimeter used in introductory physics.

FUNCTION

ACCURACY

DC Voltage

±(0.1% + 4 digits)

AC Voltage

±(0.5% + 20 digits)

DC Current

±(0.5% + 1 digit)

AC Current

±(1.0% + 10 digits)

Resistance

±(0.2% + 2 digits)

A "digit" above refers to the rightmost (least significant) digit displayed.
For example the "+1 digit" in a DC current reading "1.4869 A" would be 0.0001 A.
You could report the error in DC currents on the 2 A scale as:
The "+4 digits" in a DC voltage reading "166.51 mV" would be 0.04 mV so
you could report the error in DC voltage on the 200 mV scale as:.
However since different scales have different sized "digits", the above
formulas do not apply if the scale was changed. The SCALE function
helps with this problem if (as usual, but not universal) scale changes happen
at "2". Viz:

Thus any DC voltage reading error could be written as:
as long as all y values use the same unit (say mV). Note: I've assumed above
that y is positive, otherwise I would need to write, for example:

Functions

The functions available for your error formula include the usual fortran functions:
ABS, ACOS, ASIN, ATAN, COS,
COSH, ERF, ERFC, EXP, INT,
LOG, LOG10, NINT, RAN, SIGN, SIN,
SINH, SQRT, TAN, TANH plus GAMMA (the gamma function),
K (complex elliptic integral), NORM (the area under
the normal curve, from minus infinity to x), and INORM
(the inverse of the previous function).
The value of X, XE, Y, YE for the current datapoint are
available using the appropriate symbol (however note that if both xe
and ye are calculated from formulas, ye will still be zero when
xe is calculated). PI is a handy constant. For example,
if x is in degrees, would give
you what your calculator might record as "cos(x)". (That is fortran
trigonometric functions are designed for radian measure.)