If more than two notes are tied together, there should be a separate tie connecting every note but the last to its successor. If more than two notes are slurred together, however, they should generally all use one slur unless layout considerations force them to be placed separately (e.g. if a slurred passage doesn't fit on one line).
Although not all engravers are consistent with this, it is useful to place ties between the notes being connected, while slurs should extend between points that are above and below the notes in question. If one makes ties and slurs visually distint in that manner, there will be no confusion about whether something is a tie or a slur.
Otherwise, if has a curved mark which connects a point above or below a note that has an accidental with a note on the same staff position in the next measure, it may be unclear whether it's a tie that extends the duration of a note that sustains the altered pitch, or a slur which connects an altered note in one measure with an unaltered note in the next. If ties are placed horizontally between notes, then ties and slurs would be recognizable as different.