SIAP table import dialogue. The SSAP one looks very similar
By selecting the SIAP or SSAP option from the Load Window's DataSources menu, you can obtain a dialogue which allows you to perform a Simple Image Access or Simple Spectral Access protocol query to one of the servers on the network which supports the Virtual Observatory protocol in question.
The purpose of these protocols is to allow you to locate image or spectral data from virtual observatory servers. The basic idea is that you decide which server you want to query, specify constraints such as the region of sky in which you are interested, and then make the query. The server responsds with a table (a VOTable) which contains a row for each image/spectral data set matching the query. The load dialogues described here allow you to locate suitable servers, make simple queries along these lines, and view the returned tables in TOPCAT. This isn't the most sensible use of these protocols - typically they would be used by an image/spectrum viewer which would take care of acquiring the actual image/spectrum data described in the return table - so these dialogues are of somewhat limited use. Moreover, they only provide a subset of the options specified by the protocols (especially in the case of SSAP, which defines many possible constraints beyond sky position). However, they're here if you want to use them for whatever reason.
Operation is quite similar to the Cone Search dialogue (see Appendix A.5.4 for slightly more detail on this). First you need to select the server to use. Fill in the Registry and Keywords fields at the top of the window to restrict which SIAP/SSAP servers you want to search for, and hit the Submit Query button. If you leave the Keywords field blank, all suitable servers will be listed; at the time of writing, there are not very may SSAP servers in total, so it's not an unreasonable thing to do. Then select the server you want by clicking on its row in the table.
Having selected the server, select the region of sky in which you are interested. Either fill in the RA and Dec fields manually, or fill in the Object Name field to get it resolved to a position using Sesame. Then enter the region size - semantics of this differ slightly between protocols, see the tooltip for details. Finally, hit OK and the query will be performed, loading the result into TOPCAT as a new table.
Note that this window requires certain optional components of the TOPCAT installation, and will not be available if you have the topcat-lite configuration.