An Ensembl marker is uniquely identified by a pair of primers. This is practical for Ensembl even if it deviates from the typical biological definition of a marker. A pair of primers is used to map a marker on the genome via a marker_feature. If a marker has more than one marker feature, it will be placed at more than one position on the genome. The marker_feature.map_weight column stores the number of times the marker maps to the genome. (This is obviously done after all marker_features are created).
The marker's name in Ensembl is the display_marker_synonym_id. If a marker in Ensembl is known under different names in other databases, these are stored in marker_synonym. The same identifier might be associated with different primer pairs and will have different marker_id in Ensembl.
Ensembl markers are usually part of a mapping project that uses experimental methods to place them on the genome. A map represents a collection of experimentally placed markers and depending on the type of experiment the marker map location may have different forms (bands, radiation hybrid, cM) depending. An experimental mapping usually produces a chromosome_name. If the markers used for the experiments are not defined by a primer pair, the actual name is provided as link to the synonym table.
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