How can I select the type of relationship between fields?
FileMaker is reporting what type of relationship is possible given the field options of the field that you have selected as a match field. A field defined as an auto-entered serial number field or a field that has a unique values validation option specified will show the "single line connector" expected on the "one" side of a relationship. (global and other non-indexed fields show a "T" connector.) If non-unique values are possible, you get crows feet. This does not necessarily mean that you have to change a field option and get that single line connector as your relationship might function without any problems even though you have those Crows Feet on both ends of the relationship.
For your second question. What you describe does not make sense. Presumably, you can have many samples with the same location. That would imply a one to many relationship from Location to samples by Location ID, not Sample ID. Sample ID would not be a value that you would store in such a location table. And you would usually not want to have any fields in Samples that store the same data as a field in Location except for that Location ID value that links the two tables.
I reformatted my two tables, Location, which has a a location ID (and can have many samples), and Samples, which will has the sample IDs that correspond to the locations. I don't have the location of each sample entered into the Sample table, and so what I am trying to do is to link the location (or location ID) to that of the Location table so that it will autopopulate the location for all of the samples in the Sample table. So far I have been unable to do this however, what would you suggest so that I am able to autopopulate the location of the samples?
Actually I believe that I was able to figure it out! Thank you for the help