Effects within symbols act to dynamically alter the geometry before the graphical aspects of the symbol are applied. This dynamic geometry is only understood by the symbol itself. An example is the Dash effect, which dynamically modifies line geometry (making small segments of multipart geometry) before a stroke symbol is painted on. Some effects actually dynamically change geometry type. For example, the Buffer effect can create dynamic polygon geometry from point or line geometry. This necessitates a fill layer within the symbol to dictate how this dynamic polygon geometry will be displayed.
Since effects can act cumulatively, you can build complex symbols using the basic building blocks of symbol layers and effects. Keep in mind that effects can be applied globally at the top level of a symbol, thereby passing the dynamic geometry to all the layers in the symbol. Or, effects can be added within a single symbol layer, impacting only that one. Global effects are always applied first, before symbol layer effects.
There are no restrictions on how you structure symbols, but be aware that if a symbol is missing the correct symbol layer type to draw the dynamic geometry provided by the effects, a warning appears and the symbol may not draw as expected. To repair a symbol in this state, review the symbol in its entirety from the Structure tab 
and try to identify inconsistencies. Keep the following in mind:
- The end of a cumulative chain of symbol effects—including global effects—determines the geometry type of the symbol. A symbol layer that corresponds with this geometry type must be present. At least one marker symbol layer must be present to draw dynamic point geometry. At least one stroke symbol layer (or one marker symbol layer with a marker placement that places markers relative to a line) must be present to draw dynamic line geometry. Dynamic polygon geometry can be displayed by fill symbol layers drawing their interior, stroke symbol layers drawing their outline, and/or marker symbol layers that include marker placements that place the markers relative to a polygon.
- Conversely, a symbol layer must have corresponding geometry to draw.
- In the Structure tab of the Format Symbol mode of the Symbology pane, symbol layers are ordered with the top of the draw stack nearer the top of the pane. Those layers listed higher in the stack will draw last.
- In this same tab, symbol effects are listed in the order they are applied. Effects listed higher in the stack are applied first. Global effects impact all symbol layers and are applied before any layer symbol effects.
Caution:
Custom representation geometric effects applied to symbols in ArcMap will be dropped when the symbols are converted into ArcGIS Pro. Symbols are converted by either importing symbolized maps and scenes or by importing styles into an ArcGIS Pro project.