I have read on the official website that document checklist (imm5483e) should be added on top of application. But for online application there is no option for that.
For the online application, you'll know exactly which forms are needed because you'll be presented a table with the forms to download, fill and upload back.
In the client information form which is made optional for applicants, you can upload your SOP, and any other documents required by your visa center (for example, Rabat VAC requires a police certificate, criminal records extract, and all diplomas/transcripts).