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Hi DavidThere are two high level steps to using custom map symbols
First make the symbols available to Spectrum Spatial
Obtain a copy of the symbols (ideally in PNG or GIF format to support background transparency)
In Spectrum Spatia2018.2 you can create and add symbols to a custom symbols folder. This folder can be configured to be outside of the install path so it is retained when upgrading
In Spectrum Spatial 12.1 and prior symbols are copied into the install path under lib.
Second - style override your layers to reference the symbols.
This can be done either (1) in MapInfo Professional and then the layers are uploaded the named resources (Named Layer) will include the style override or (2) in Spectrum Spatial Manager 2018.2 by modifying thee layer and adding a style override
Not that style override is preferable to editing TAB files and applying the styles to the data itself. You can also use individual value thematic maps to style different data with different symbols based on a column in the data.