# properties.templateId.description
Send using a saved template — a user template (tpl_…) or a base template (base_…). Its subject/html/text seed the message; explicit subject/html/text here override them.

# properties.templateData.description
Values substituted into the template's {{merge_tags}} (e.g. {"name":"Ann"} fills {{name}}). HTML values are auto-escaped.

# properties.headers.description
Extra raw MIME headers, applied after threading headers (caller wins). Use for what the structured fields can't express — e.g. `List-Unsubscribe`, a dedup/idempotency key (`X-Entity-Ref-ID`), or a tag header (`X-Tag`). Carried on both immediate and scheduled sends.

# properties.attachments.items.properties.url.description
Fetch the file from this URL at send time and attach it. Any URL works; an uploadAttachment() URL is the secure, recommended choice. Prefer this over `content` for large files.

# properties.attachments.items.properties.content.description
Inline file bytes as base64. Simple for tiny files, but re-uploaded on every send — use `url` (see uploadAttachment) for anything large.

# properties.attachments.items.properties.contentType.description
MIME type; defaults to application/octet-stream.

# properties.scheduledAt.description
Send later: ISO 8601, simple relative natural language ("in 2 hours"), or a ms-epoch. A future time parks the message with the scheduler — the response carries an ssnd_… id and status "scheduled", cancelable via DELETE /v1/scheduled/{id}. Omitted or past sends now.

# properties.trackOpens.description
Open-tracking override for this send (HTML only). Omitted → the from-domain's default applies.

# properties.trackClicks.description
Click-tracking override for this send (HTML only): http(s) links are rewritten to a signed redirect that records the click, then 302s to the destination. Omitted → the from-domain's default applies.