For radiation from a laser, what part of the EM wave is interesting?

Question

On page 1 of the book "Principles of Lasers and Optics” by William Chang, the author, speaking about radiation from lasers, says that:

For devices with structures that have dimensions very much larger than the wave-length...

...we look for a simple analysis which can give us just the main features (i.e. the amplitude and phase) of the dominant component of the electromagnetic field in directions close to the direction of propagation and at distances reasonably far away from the aperture.

I don't understand this sentence, it may be possible that I'm misunderstanding what the author is telling us.
In particular, why is the dominant component of the EM field along the direction of propagation important? For example, if we consider a TEM mode, the component along the propagation direction is zero.
Second, what is "the aperture"? It's the first time in the book that the author use this term.

Thomas Fritsch · Accepted Answer

"Aperture" is just another word for opening,
especially the opening in optical devices.
Consider this image of a wave going through
an aperture and getting diffracted:

(image from Wikipedia - Diffraction)

For devices with structures that have dimensions very much larger than the wavelength

As you probably know:
When the object structures are not much larger than the wavelength,
then you get very pronounced diffraction.
And when the object structures are much larger than the wavelength,
then you get only small diffraction effects.
In the image above the aperture size is 4 times the wavelength.
And because 4 is not a very big number, the diffraction is quite pronounced.

we look for [...] the electromagnetic field
in directions close to the direction of propagation
and at distances reasonably far away from the aperture.

Applied to our example this means:
We are only interested in the parts near the main central beam
and far away from the aperture (the area marked in green in the image below).
Restricting to these parts allows for simpler mathematics
when calculating the wave field.

For radiation from a laser, what part of the EM wave is interesting?

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