Tone is the attitude a reader perceives through your wording, sentence structure, level of detail, and degree of directness. The right tone depends on purpose, relationship, urgency, channel, and organizational culture. Professional writing is not always formal, but it should remain clear, respectful, and appropriate to the situation.
Direct language can still be warm, and polite language can still be firm. State the facts, focus on actions rather than personalities, and use proportionate courtesy. Avoid adding so many softening phrases that the request disappears, or using abrupt wording when context and empathy are needed.