It's a little confusing at first, because they're all chameleons really.
What color is a chameleon? It's not simple.
BYLAYER color and linetype, when in a block inserted on a layer "A", will
like the BYLAYER color and linetype of layer "A". It will look
different inserted on layers that look different. Considerable flexibility.
BYBLOCK color and linetype, when in a block inserted on a layer "A", will
like whatever the current color and linetype setting are. (They can
also be changes later whenever you like.) If the current color and linetype
are both BYLAYER, it will look exactly like the BYLAYER example above, but
if you change the block's color and linetype the BYBLOCK elements in it will
change. You could make it look the same on any layer if you want, or even
different from on another on the same layer. It is most flexible.
Assigning color, linetype, and layer(s)-other-than-0 will mean that the
block will always look exactly the same, no matter where it is inserted, and
you will only be able to control its visibility by freezing the insert
layer, or the layer(s)-other-than-0. No flexibility.
Layer 0 is meant to be used for elements in blocks that you wish to behave
exactly as if they were made on the layer on which the block is inserted,
from the point of view of visibility. If a block is made of all layer 0
stuff, inserted on layer "A", and you turn off layer "A", the block will
disappear. If the block elements were on layer"B", and you insert it on
layer "A" (and layer "B" remains on and thawed) they would remain visible if
you turn off layer "A" (because they are on "B", and the block insert layer
is not being ignored). Freeze "A" and it will disappear because when you
freeze the layer of an insertion, the whole block and al its contents is
ignored. Turning a layer off only asks that elements on that layer not be
displayed, and in the case where the block elements are on layer "B", only
the block's insertion point is on "A".
I suggest you play around with the various permutations if it still isn't
clear. Remember that FREEZE and OFF are not the same thing. OFF will
unclutter you screen to some extent, depending on the way the drawing is
structured, by not rendering elements on that layer. FREEZE will not
recognize or render anything associated with a layer and speed up your
In my business, most people draw using BYLAYER as a default, and therefore
think that the main reason for layering is to make different-looking stuff.
I use a combination of all the above strategies to get the most flexibility
in my blocks and prettier plots, and use layering for visibility control.
Click to see the full signature.